Iron Frogs: 8-inch Model 1861 Siege Mortars

Once again, let me return to one of the photos from Broadway Landing:

This time, let me call attention to the squat mortar in front of the row of howitzers:

Like an iron frog, just sitting quietly.  Waiting for a chance to grab a passing fly.

It is/was an 8-inch Siege Mortar, Model 1861, made to the new pattern established by the Ordnance Board the year before.  So first let’s compare the Model 1861 to the previous production types discussed earlier as the Model 1840.

With the addition of 85 pounds in weight, the Model 1861 added a bit of length to the bore.  The weight and design also allowed for a small increase in powder charge.  This allowed for a modest increase in range, according to the firing tables.  But those statistics cover up several improvements, best described through the “walk around.”

Perhaps the most important statistic in the chart is the number produced – over four times the number of Model 1840.  Remarkably more than half, at present count nearly 90, of those produced survive today.  And the survivors offer plenty of opportunity for detailed, up-close examinations… and discussion of those improvements.

As the chart indicates, three vendors produced the 8-inch Model 1861.  The first to commence deliveries was Cyrus Alger of Boston.  Thomas J. Rodman inspected the first, credited in April 1862, of a batch of ten.  Rodman also inspected the second batch of 17 delivered between October 1863 and February 1864.  There’s another Rodman connection beyond the inspection stamps.  The second batch (and possibly subsequent castings) used the Rodman method of hollow casting.  Later Alger delivered twenty more, bringing their total to 47, which were inspected by other officers.

Fort Pitt Foundry commenced production of the type in late 1862 with initial deliveries in January 1863.  As with Alger, contract specifications for those cast in 1863 called for hollow casting.  Fort Pitt delivered its last of the type in September 1864, completing a 73 example run.  I mentioned one of those Fort Pitt mortar in an earlier post about the guns displayed at Sunbury, Pennsylvania.

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8-inch Siege Mortar Model 1861 at Sunbury, PA – Fort Pitt

Muzzle markings offer details of the weapon’s manufacture:

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Muzzle of Fort Pitt #24

This mortar is registry number 24 from Fort Pitt.  It was accepted by Robert Henry Kirkwood Whiteley (R.H.K.W.) in 1864, weighing 978 pounds.  On the other side of the display is the next weapon produced in the registry sequence – number 25.

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Muzzle of Fort Pitt #25

Notice the raised numbers above the right trunnion on these mortars.

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Fort Pitt #25

The raised numbers were the foundry number, a unique practice used by Fort Pitt.  This number was often repeated on the right rimbase, in accordance with regulations.

And speaking of the trunnions, compared to the previous model, the trunnions in this year model moved from behind the chamber to the center of balance.  This was due to the use of a zero-preponderance elevation system, somewhat similar in concept to that used on the larger seacoast guns and mortars.  The mortars had a raised strip with rectangular sockets.  This allowed the use of an elevating bar, again just as with the big Rodman guns.  There was some deviation in the number of sockets.  Early Alger production 8-inch Model 1861 had three sockets.  Later they increased the number to eight.  Fort Pitt mortars had three.

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Top profile of Fort Pitt #25

There was some variation in the trunnion length also.  Alger production batches had trunnions 3.25 inches long.  Early Fort Pitt production had trunnions barely over 2 ⅓ inches.  The foundry increased the length to three inches and then up to the full 3.25 inches as production continued.  Those at Sunbury are behind an iron fence, but appear to be 3 inches long.

Notice also the lifting lug on top of the mortar.  This was yet another addition to the 1861 design.

Seyfert, McManus & Company, working from the Scott Foundry in Reading, Pennsylvania, produced 50 examples starting in August 1864.  All of their production came under a contract calling for hollow casting.  Two of those sit today in a cemetery in Lewisburg, Pennsylvania.

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Mortars and Memorial at Lewisburg

These are registry numbers 33 and 49, both accepted in 1865.  The mortars from Reading featured nine sockets on their breech.

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Breech of Seyfert, McManus mortar

Seyfert, McManus & Company mortars used the full 3.25 inch long trunnions.  Notice the absence of the lifting lug in this view.  These are mounted inverted and the lifting lug is underneath now.

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Lug on Inverted Mortar

Such prevents easy view of the vents.  The Model 1861 design called for two vents – one on the left initially drilled through and on the right with the last inch undrilled.  The mortars’ vents were not bouched.  When the left vent wore open with use, the artificer closed it with zinc.  He then drilled out the last inch of the right vent to put the weapon back in service.

The 8-inch Model 1861 Siege Mortar entered service as the “light” weapon in the family.  Wartime photos indicate that, in addition to work on the Richmond-Petersburg lines, the mortars saw service in the Washington defenses. Long after the war these weapons remained in depots as part of the Army’s siege train.  John C. Tidball included a section detailing the service of an 8-inch siege mortar in his 1884 Manual of Heavy Artillery Service.

Yet outside the intense period of 1864-65, the 170 8-inch Model 1861 Siege Mortars spent much of their time idle.  Perhaps, due to that long, uneventful service life, there are many of these “iron frogs” surviving as gate guards and memorials.

Next up, the 10-inch Model 1861.

Fort Macon, NC - 10 In Siege Mortar Model 1861
10-inch Model 1861 Siege Mortar

Photo Examination: 10-inch Mortars at Dutch Gap

In an earlier post, I discussed the 10-inch seacoast mortars in this photo:

The photo is remarkable at several layers.  I suspect the photographer captured work being done in July 1864.  The photo caption carries the caption “Work party and mortars at Butler’s Crow’s Nest.”  That leads me to dispatches appearing in the Official Records (specifically Series I, Volume 40, Part III, Serial 82, pages 23-24) from Captain Alfred Mordecai (this would be the “Junior” and not the more famous father).  On July 5, 1864, Colonel Henry Abbot, 1st Connecticut Heavy Artillery, directed Mordecai to prepare the “Crow’s Nest Battery” with a “Sawyer gun” and a 100-pdr Parrott.  Later that day, Mordecai added that two 10-inch mortars were ready for mounting at the Crow’s Nest.  These dispatches, and the presence of the signal tower in the background, indicate the photo captures work done opposite Dutch Gap, before the famous canal project.

Another photo, this one depicting Battery Sawyer, appears in the Atlas of the Official Records.

The image was part of a report forwarded by Lieutenant Peter S. Michie in a report dated September 1864.  The gun on the parapet may well be a Sawyer gun (a type I have not gotten around to detailing in a post).  The sub-caption reads, “10-inch Mortar Battery.”  While this leaves open speculation as to the exact battery location, at least there is some corroboration.

Mortars at this battery saw action against neighboring Confederate works.  On January 24, 1865, the mortars fired on Confederate warships during the battle of Trent’s Reach.

As for the mortars themselves, thankfully the photo’s resolution allows for interpretation of the markings.  The mortar with the muzzle facing the camera is registry number 7, with a recorded weight of 5800 pounds.

The weight leaves no doubt this is a 10-inch Seacoast Mortar Model 1840 (the Model 1861 in the same caliber weighed significantly more).  There were two vendors producing the Model 1840 – West Point and Cyrus Alger – and thus two possible registry number 7s.  Stephen Vincent Benét inspected the West Point weapons.  Our old friend Thomas J. Rodman inspected those from Alger.  And a close examination of the inspector’s initials on the bottom of the muzzle shows…

… T.J.R.  That means this particular mortar was one of five produced between May and October 1861.  The mortar to the right lends additional evidence for the vendor.

No mistaking that: Boston.  As noted in the earlier article, none of the Alger mortars are among the known surviving pieces today, unfortunately.  But at least this photo verifies the weapon’s existence and use.

The photo also provides a chance to examine the seacoast mortar bed in detail.  One of two officers in the picture was leaning against a mortar bed awaiting its station.

Note the squared bolt heads for the cross members.  Great detail if you plan to rebuild one of these at some point.

But the real interesting part of this photo, in my opinion, is what is going on behind the mortar bed.  Look closely at the workers.

The two men in the center frame were certainly African-American. The men to the left and right (closer to the photographer) were lighter skin, beyond a doubt Caucasian.  Three of the four men in this section have tools in hand.  So does this photo capture a mixed work party of USCT and white troops?   Contrabands and white soldiers?

I’m sure the photographer had some objective in mind when framing this scene.  He wouldn’t waste valuable plates on just some ordinary digging operation, even if he could foresee my excitement at identifying the mortars.  So was it the mortars?  Or was it the mixed-race work party?

The Big Rodmans: 20-inch Rodmans, Part 2

In yesterday’s post on the 20-inch Rodman Gun, I’d mentioned the gun pictured below arrived at Fort Hamilton, New York for testing in the fall of 1864.

20-inch Rodman #1

Compared to the trials of the 15-inch gun, the 20-inch gun received a bit more fanfare – and full coverage in Harper’s Weekly. When first fired on October 26, 1864, those in attendance included the gun’s inventor Major Thomas J. Rodman; Army Chief of Ordnance, Brigadier General Alexander B. Dyer; Captain Henry Augustus Wise, Chief of the Navy’s Bureau of Ordnance; and General W. F. Smith. If illustrations accompanying the Harper’s article are any indication, a rather large crowd of onlookers joined the dignitaries.

“Large” was the word for every aspect of the gun. The gun rested upon a specially built 18 ton carriage from the Watertown Arsenal, which was also designed by Rodman. As a measure of the size of the gun, prior to firing, a man lowered himself into the bore of the gun and wormed down to the vent in order to clear an obstruction. 1

The objective of the test was only to verify the gun could fire the heavy projectiles. Unfortunately I’ve never come across any first hand account or report of the gun’s trials. Most secondary sources point to a caption in Francis Miller’s Photographic History of the Civil War. Although Miller incorrectly identified the photo of the 15-inch prototype as the 20-inch gun, he stated the big gun fired four shots in 1864. Firing a 1,080 pound solid shot, the gun used charges increasing through 50, 75, 100, and finally 125 pounds of powder. According to Miller, tests resumed in March 1867 with four more shots. This time charges of 125, 150, 175, and then 200 pounds, fired at a 25° elevation, propelled shot to a maximum range of 8,000 yards.2

Certainly impressive figures, but with the war winding down that massive iron gun was too much for peacetime budgets. Fort Pitt delivered one more 20-inch gun in 1869. This gun also survives today, on display at Fort Hancock, New Jersey. (And again let me thank Bill Coughlin for the photo.)

20-inch Rodman #2 at Fort Hancock

This gun bears the registry number 2 along with its weight marks of 115,100 pounds. John Alexander Kress inspected this monster gun. Once accepted, #2 went to Fort Monroe. In 1876 the gun went to Philadelphia for the Centennial Exposition.3

20-inch Rodman at Centennial Exposition

From there the gun eventually ended up at Fort Hancock among other heavy guns also undergoing trials in the later decades of the 19th century. Thankfully it was preserved to “guard” the post instead of being discarded.

Although the Army received only two 20-inch Rodmans, the guns significantly influenced post-war planning. Plans considered these largest of guns for the critical locations along the seacoast. Over the years, schemes for the 20-inch guns included more advanced carriages, muzzle loading rifled variants, and even breechloading conversions. None of these progressed far. Army acquisition at the time focused on refinement of weapons which could be mass produced in the event of war (since everyone felt there’d be ample warning before any future war). As such, the 1864 testimony of William Wade, part owner of the Fort Pitt Foundry, is worth note. When asked by the Joint Committee on the Conduct of the War, Wade indicated his foundry’s capacity was, “two 10-inch or 8-inch guns per day, three 15-inch guns per week, and two 20-inch guns per month.4

Perhaps we could end the story there, simply saying the two 20-inch Rodmans were great guns that were never called to action. But that’s not the case. In the late 1870’s Peru came shopping for weapons due to heated tensions with Bolivia and Chile. In addition to some of the 15-inch Rodmans, the Peruvians acquired at least on 20-inch Rodman (sometimes noted as a 1000-pdr gun), presumably a surplus weapon at the foundry. During the War of the Pacific, this Rodman, paired with a 20-inch Dahlgren gun also cast by Fort Pitt, was used in defense of the port of Callao. 5 The guns presumably wound up in the hands of the Chileans at that point. Perhaps the big Rodman guns did fire more than a few “test shots” after all.

The legacy of the 20-inch Rodman was to American seacoast defenses in the last decades of the 19th century. While the days of muzzleloading, black powder, smoothbores waned after the Civil War, the range of the 20-inch guns influenced plans. With these larger guns, defenders could control larger expanses of waterways. Instead of just defending the harbor entrances, the Coast Artillery could think about covering likely approaches to the coast. The real change would wait until modern breechloading, rifled, steel guns arrived.


  1. Harper’s Weekly, Volume VIII, No. 412, November 19, 1864, page 749.
  2. Miller, Francis Trevelyan, The Photographic History of the Civil War: Volume 5 – Forts and Artillery (New York: Review of Reviews, 1911), page 137.
  3. Ingram, J.S., The Centennial Exposition Described and Illustrated (Philadelphia: Hubbard Brothers, 1876), page 146.
  4. Testimony of William Wade, “Heavy Ordnance,” Report of the Joint Committee on the Conduct of the War at the Second Session, Thirty-Eighth Congress (Washington: Government Printing Office, 1865), page 86.
  5. Markham, Sir Clements R., The War Between Peru and Chile, 1879-1882 (London: Sampson Low, Marston, and Company, 1882) page 186.

Less successful sibling: The 13-inch Rodman Gun

I’ve detailed the design and production histories of the 8-inch, 10-inch, 12-inch Rifle, and 15-inch Rodman Guns.  Before I turn to the “big brother” of the set, the 20-inch Rodman, for sake of complete coverage I should mention the 13-inch Rodman smoothbore gun.

Thomas J. Rodman’s 1861 patterns included the 13-inch gun with proportions between that of the 10-inch and 15-inch guns:

Fort Pitt Foundry cast one 13-inch Rodman in mid-1863, the only gun of that caliber cast to Rodman’s original pattern.  Given the time frame, the gun likely had no preponderance and sockets for the updated elevating system.

Two more 13-inch guns appear in the records, but differ from the original pattern.  In 1866, Cyrus Alger delivered a pair of 13-inch Rodmans.  The first used the mold of a 15-inch Rodman, but bored to the smaller caliber, weighing over 51,000 pounds.  The exact pattern for Alger’s second is unknown, but it weighed 38,500 pounds.  The heavier of the pair went to the test range for trials and burst after firing over 700 rounds.  Testimony attributed the failure to tests of quick burning powder, which generated many times the pressure of the mammoth powder used in service.

Very likely the third 13-inch Rodman became the subject of a different experiment.  An 1878 report to Congress details the conversion of a 13-inch Rodman smoothbore to a 10-inch rifle.  Starting with a 13-inch gun cast in 1866, South Boston Foundry, which Alger’s facility became in the 1870s, reamed out the bore to 17 inches.  The foundry then installed a wrought iron rifled bore and jacket.  The rifling pattern included seventeen grooves each just under one inch.

The product weighed 40,320 pounds.  The weight is within the range expected if the second of Alger’s guns was the subject of conversion.  But the report does not offer a foundry or registry number for conformation.  Regardless the gun went to Sandy Hook, New York for tests.  Like many expedients of the late 19th century, the conversion offered little gain for the money expended.

While not produced in numbers, the 13-inch Rodman appeared in post-war plans.  In 1867 a board of engineer, artillery, and ordnance officers recommended the 13-, 15-, and 20-inch as the preferred smoothbore calibers.  The recommendation apparently carried weight, for as late as 1891 the Manual for Heavy Artillery listed 13-inch smoothbores as “in the system.”  Although General John C. Tidball noted it should be considered an “experimental type” as only a few were produced.  In some regards this reflected American strategic thought at the time.  Should war come again, the Army and Navy would have ample time to re-arm with the preferred weapons.

I would question why the 13-inch gun ever existed to begin with.  Certainly the 15-inch gun provided greater firepower.  In the context of coast defense, what part entrances could the 13-inch cover that the 10-inch could not?   Particularly with the coastal forts of the time positioned with the older seacoast guns in mind.  Then again I was not an ordnance officer in the 1860s!

None of the 13-inch guns survive today, leaving us with a few line drawings and congressional testimonies to tell their story.  And that story is but a footnote in the larger chapter of the Rodman guns.  The 13-inch guns were the unsuccessful sibling of the family.

Walk Around the Lincoln Gun: 15-inch Rodman Prototype

The last several artillery posts have focused on the gun in this wartime photo:

15-inch Rodman Prototype at Fort Monroe

The 15-inch prototype remained at Fort Monroe after trials ended, very successfully, in early 1861.  Worth noting, the gun received its first nickname around that time – “The Floyd Gun” – after Secretary of War John B. Floyd, who’d been a supporter of the project.  By April 1861 the Fort Monroe garrison mounted the big gun on a barbette carriage near the Old Point Comfort lighthouse.   I’d speculate the carriage was either the same or a similar outfit to that used during the proofing in 1860.  However, the gun didn’t sit there very long.  In July 1861 the new 12-inch Rodman Rifle arrived for testing.  Jumping the gun, so to speak, General Benjamin Butler ordered the untested rifle onto the 15-inch smoothbore mount.

After the USS Monitor-CSS Virginia duel, General John Wool ordered the 15-inch gun placed back in a barbette carriage into the fort’s defenses.   The circular traces used by one of these two guns remains today, ironically, in Jefferson Davis Park on the fort’s wall.

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Traces for Center Pivot Barbette Carrige in Jeff Davis Park

On April 15, 1862, both the 12-inch rifle and the 15-inch prototype gun fired in the direction of Sewell’s Point, more so to gain the range than against any particular target.  But with this activity one can say the prototype “fired in anger.”

It was also around this time the gun received a new nickname.  With Mr. Floyd’s resignation and subsequent commission as a Confederate general, the old name was just bad for public relations.  So Secretary of War Edwin Stanton ordered the gun’s name changed to “The Lincoln Gun” and it remains so today.

If the wartime photo’s listing data from the National Archives is correct, the 15-inch prototype remained in place through 1864.  We know it is indeed the prototype from the muzzle markings.

Close Up of Muzzle

At the top appears the registry number  “1” and at the bottom are the initials “T.J.R.” for Thomas J. Rodman.

The right side trunnion, although at an angle in the photo, clearly shows the foundry stamps.

Right Trunnion in Photo

The stamps read “F.P.F.” and “K.R. & Co.”  These stand for “Fort Pitt Foundry”
and “Knap, Rudd and Company” respectively.   That particular stamping set appeared only in 1860.

Today the Lincoln Gun rests only a few hundred yards from its wartime post, at the edge of Fort Monroe’s parade ground.

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15-inch Rodman Prototype

My photo of the muzzle markings didn’t turn out right (for which I’ve kicked myself several times).  So trust me that the “No. 1” and “T.J.R.” remain on the muzzle.  The right trunnion also still shows the manufacturer’s stamps.

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Right Trunnion of Prototype Rodman

And the left trunnion provides the year of manufacture.

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Left Trunnion of Prototype Rodman

Notice the trunnions are shorter than those of earlier Columbiads.  Rodman designed the 15-inch gun to fit iron, not wooden, carriages.  However the prototype’s trunnions are about a half-inch longer than those of production Rodmans.

The breech displays the weight, just as recorded by Rodman in May 1860.

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Weight Stamp on Breech

Compare the ratchets in the breech face to the sockets on the 15-inch guns at Fort Foote, Maryland.

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Breech Face of Production 15-inch Rodman

Less easily determined by the naked eye is the shift of the trunnion location.  Production 15-inch guns had the trunnions just 1.25 inches to the rear, eliminating preponderance.

Rarely can we trace the story of a Civil War era gun with any degree of certainty.  However in the case of the Lincoln Gun, we know every place the gun was from foundry to field site.  We even have reasonably accurate records of every time the gun fired!

The Lincoln Gun – an artifact preserved in Fort Monroe National Monument with a story to tell!

15-inch Rodman Prototype: Trials of the New Gun

Discussing the prototype 15-inch Rodman gun, I left off with the 25-ton iron form arriving at Fort Monroe for trials.  Now it’s time to talk about shootin’ that big gun.

Thus far in the discussion of Rodman guns, my articles focused on “experiments” within the scientific sense.  But prototyping takes on a different focus.  To pass trials, a prototype undergoes tests to ensure it meets requirements and expectations.  Beyond that, prototype testing ensures the system is a practical fit in the desired role.  The focus extends more to “will it work the way we intend to use it?”

Instead of placing the gun on very restrained test mount, Captain Alexander B. Dyer had a wrought iron carriage built at Fort Monroe Arsenal for the prototype.  Generally, the carriage matched that of the Columbiad barbette center pintle design – just bigger all around to fit the bigger gun.  Recall too the prototype 15-inch Rodman retained a 1200 pound preponderance at the breech along with the ratchet elevating system.

For trials in 1860, the crew at Fort Monroe placed the gun on a concrete and cut stone platform along the beach.  As with the standard service mounting in a fort, the trials mount had concentric rings to allow the carriage to traverse.  The mount allowed just over 28° elevation.

Normally any new gun undergoes proofing to verify it could withstand service charges.  However, since the 15-inch was an entirely new design proofing involved testing to see just how much load the gun could reasonably withstand.  Proofing with 0.6-inch grained powder started with 25 pound charges propelling a 330 pound shell.  After five shots this increased to 30 pounds with a 310 pound shell, which the gun fired five times.  At subsequent intervals, the powder charge increased by five pounds until reaching 40 pounds.  Rodman also fabricated several “perforated cake” charges for these tests (and those deserve treatment in a separate post). During these proofs, the greatest range was with a 40 pound charge at 28° elevation reaching 5730 yards.

After forty fires during the proof, a board of officers, representing the engineers, ordnance corps, and artillery, determined the gun was ready for evaluation.  Just to drop names, the board included General Joseph Totten, Major John Barnard, Captain Horatio Wright, Major John Symington, Captain Dyer, Captain Josiah Gorgas,  Colonel Justin Dimick, Major Robert Anderson, Captain J. H. Carlisle, and Lieutenant G. Tallmadge.   For the board, the gun fired forty-nine more times at an elevation of 6°, with a 35 pound charge and a 317 pound shell.  Ranges varied between 1873 and 2017 yards.  During these fires, the upper carriage recoiled between 68 and 77 inches.

Rodman attempted to measure muzzle velocity with a Navez device, but results were erratic.  Instead, the crew fired a few rounds at a short range target (885 feet, because “short range” is relative) and estimated velocity from the time of flight. With 35 pounds of powder, observers figured velocity at 1328 feet/second.  Using 50 pounds of perforated cake, velocity dropped to 1282 feet/second.  Interestingly, these tests used rope grommets instead of sabots.

After a few more ranged shots, the trials got down to some fun trials – ricochets on the water!  The board wanted to know the behavior of the large projectile when used in one of the favored anti-ship techniques of the time.  With elevations ranging between horizontal and 5°, using 40 pound charges and 318 pound shells, five shots fired over the open waters around Fort Monroe.  However due to rough waters, these tests were far from conclusive.  Yet the conclusion was higher elevations yielded better ricochet patterns.

At the end of all these fires, the board examined the bore of the gun.  Using a star gauge, the officers found no enlargements in the bore.  For all practical purposes, the gun stood up well to use.  However, the carriage had sustained some damage.  The original traversing wheels cracked under the strain of firing, and were replaced by a stronger set.

The trials also considered how the crew operated the gun.  Obviously with a larger projectile than any weapon then in service, handling was a consideration.  Rodman’s report noted that three men could load the gun – two carrying the projectile and one pushing it into the muzzle with the rammer.  However, a team of five was preferred.  Running the gun into battery required seven men, and an eighth used to ease the work.  Although two men could traverse the gun, the preferred number was four.

For a shot at horizontal, time required for servicing and loading the piece fell to just over a minute after practice.  Running the piece out at maximum elevation required between three and four minutes.  The crew could traverse the gun at a rate of roughly 45° per minute.  Not bad for the “first run” of the gun.  Even in the 1890s with crews intimately familiar with the carriage and gun, officers planned for one shot every four or five minutes in combat.

At the end of these trials the board gave the gun positive marks.  The gun survived the trials and performed well.  Despite the size, the gun was just as well handled as the contemporary Columbiads.  The officers believed, “… the introduction of guns of much larger caliber than any now in the service, is desirable and practical.”  They went on to say, “the efficiency of our present armament for harbor defense would be improved by the addition of a judicious portion of guns of this class.”  However, the board did express caution about the endurance of the gun, asking for additional fires.

Starting in December 1860, Dyer supervised additional tests on the 15-inch prototype.  All told, he exceeded 500 fires.  At the end of which, he reported no measurable wear in the bore.  Vent erosion was less than normal for a gun at that endurance.

While Dyer fired this big cannon, the country was falling apart.  I find interesting how members of the board were soon swept up in events.  Anderson, of course, would move from this trial to his next post at Fort Sumter.  Within a few months, Gorgas resigned his commission and became the Confederate chief of ordnance.  But Rodman left the trials and returned to Pittsburgh.  There he worked on a 12-inch rifled gun constructed in the same manner as the 15-inch gun.

One final note, perhaps bringing this story from one about “cannons” to one about people – describing the gun crew at one point, Rodman indicated the composition to be “one sergeant and six negroes.”  Such raises several questions.  Why wasn’t a detail of artillerymen used?  Were these freedmen or slaves?  Were these men trained in artillery handling before these trials?  Regardless, I find an interesting thread here.  On the eve of the Civil War the Army used African-American labor to test its most important new weapon.  And that very gun later acquired the nickname “The Lincoln Gun.”


Thomas J. Rodman’s report of the 1860 prototype trials appears in Reports of Experiments on the Properties of Metals for Cannon, and the Qualities of Cannon Powder; with an Account of the Fabrication and Trial of a 15-inch Gun (Boston: Charles H. Crosby, 1861), pages  281-293.

15-inch Rodman Prototype: The Result of All Those Experiments

Having discussed the early casting experiments by Thomas J. Rodman to refine his casting technique and subsequent gunpowder tests, it is time I turned to the prototypes of the Rodman Guns.  Instead of constraining the next tests to just 10-inch weapons, the first prototype of the improved pattern increased to 15-inch.  The result was this gun – with registry number 1 – the prototype of a long line of Rodman guns.

The Lincoln Gun - Fort Pitt Foundry #1

Concurrent with the gunpowder experiments, Rodman “ran the numbers” to determine the optimum thickness of metal for a 15-inch gun.  I’m reluctant to walk through the reasoning here, due to space and my “physics for poets” background.  The short version, for those of us willing to neglect the particulars of square roots and other higher forms of math, is the gun needed a thickness of around 16 inches around the seat of the charge.  The thickness increased to 25 inches from the bore bottom to the exterior breech face.

Rodman also took the time to determine, based on his complied data, the best profile for the gun’s chamber (or bore bottom as he called it).  The preceding experiments indicated the traditional flat-bottomed bore tended to crack at the corners.  In the course of experiments Rodman found the sub-caliber chambers of the old Columbiads detrimental to performance.  He wrote, “There should be no angles, either salient or re-entrant, in the termination of the bore, but he surfaces of the bore and of its termination should be tangent along their lines of junction…. the semi-ellipsoid is believed to be the best and true termination.”  However, the casting plan diagram showed that of a hemisphere.

Casting Diagram and Plan of 15-inch Prototype

The diagram indicates the use of the Rodman neck vice the traditional cascabels.  The thin lines around the finished gun’s form indicate Rodman planned for a large amount of excess metal.  On surface examination, this resembles the casting technique developed by John Dahlgren for his naval guns.

Another point of similarity, which Dahlgren would later cite, is the exterior form.  Yes, the exteriors of Rodman and Dahlgren guns feature blended curves, lacking external fixtures.  But Dahlgren’s pre-war shell guns show the use of a pure cylinder over the chamber, traditionally the location of the first reinforce on the gun.  Rodman guns, as seen on the prototype plan, featured a near continuous curve expanding from the breech face to a maximum thickness over the seat of the charge, thence gradually tapering down to the muzzle.

The exterior form matched that called the “ordnance shape” and offered few right angles where stress would accumulate.  Rodman guns offered no “flats” save that of the muzzle face, rimbases, and trunnions.  If this form borrowed from Dahlgren, the paper trail has yet to be established.  On the other hand, simple examination of Army ordnance from 1841 through 1861, considering the columbiad trials and even the shape of the 1857 “Napoleon,” demonstrate the evolution to such blended curves.

In the fall of 1859, Rodman tested and selected the best iron from supplies available at Fort Pitt Foundry.   The foundry lit furnaces on December 23 to start the casting process.  As with previous hollow core castings, after pouring the molten iron, the foundrymen  poured water into the insert.  According to Rodman’s notes, water entered the insert at 36° and exited at 58°.  Rather low temperatures considering the last 10-inch casting initially had water entering at 80° and exiting at 102°, but in the warm month of August.  Notice the difference between entry and exit temperatures remained 22°, summer or winter.

After twenty-one hours, the water temperature exiting the insert dropped to 47°, and the workers removed the insert.  As with the earlier castings, water then poured directly into the interior bore.  Exit temperatures jumped to 86°.  Cooling continued for over 140 more hours.  All told the gun sat in the pit for 168 hours, far less than the time taken to cool a solid cast gun of half the size.

The finished gun weighed 49,099 pounds, with a 1200 pound preponderance at the breech.  It measured 190 inches long with a maximum diameter of 48.1 inches.  The gun measured 25 inches in diameter at the muzzle.  The 15 inch bore ran 156 inches deep.  Regardless of the metric, this was a large gun – indeed the largest produced up to that time in the United States.

In May 1860, the gun went to Fort Monroe for trials.  And how was a 25 ton cannon moved from Pittsburgh to Old Point Comfort?

For this purpose two strong trussed beams, 50 feet long, were prepared.  These beams were placed parallel to each other, and about 36 inches apart, their ends resting upon two bolsters placed transversely across the middle points of two 8-wheeled platform cars.  The gun was suspended under the two trussed beams, and between the cars; so that its weight was equally distributed over the 16 wheels of the two cars.

Thus packed, the gun moved on the Pennsylvania Central Railroad, Northern Central, and finally the Washington Branch.  In Washington, D.C. the gun, still on the cars rolled onto a heavy cargo vessel, which took the setup to Fort Monroe.

Once at Fort Monroe, Army personnel prepared the gun for proofing.  It was time to fire some large projectiles from one of the world’s largest guns.  I’ll discuss the testing and proofing of the 15-inch prototype next.


Thomas J. Rodman’s report of the 1859 experiments appears in Reports of Experiments on the Properties of Metals for Cannon, and the Qualities of Cannon Powder; with an Account of the Fabrication and Trial of a 15-inch Gun (Boston: Charles H. Crosby, 1861), pages 192-274, 281-282.