Need heavy guns on the Potomac: Seacoast defenses for Washington

Even after all direct threats to Washington, D.C. abated with the end of the Gettysburg Campaign, at least one man in the capital city saw the need to improve defenses.

On September 1, 1863, Brigadier-General John G. Barnard wrote to Secretary of War Edwin M. Stanton:

The works of Rozier’s Bluff, and near Jones’ Point are nearly ready to receive guns–in fact they could have been mounted some time ago, had the guns and platforms been available. You are well aware that not only are the large seaport towns, like New York, Boston, and Philadelphia making strenuous exertions to increase their armament of improved guns, but even places of a (comparatively) secondary importance, like Portland, &c. If we have war with a maritime power (a possibility which incites all these preparations), the land defenses of Washington will prove unavailing unless also the access by water is prevented.

There is not now a gun mounted for the defense of the Potomac capable of having the slightest effect upon an iron-clad vessel. As it seems to devolve upon me to represent the necessities of Washington, I would recommend that among the guns which actually do become available, a fair proportion should be assigned to Washington.

The Ordnance Department is doing all that can be done to furnish guns. It has no voice, however, in their distribution, and as there are no Governors of States or commissions of citizens to advocate the needs of Washington, I feel called on to make this representation.

A few days later, Barnard would recommend the fortification on Rozier’s Bluff, on the Maryland side of the river, receive the name Fort Foote in honor of Rear-Admiral Andrew H. Foote, who died earlier in the year.  The battery near Jones’ Point was named Battery Rogers after Captain G.W. Roders, killed in action off Morris Island in August.

Not finished petitioning for heavy guns to defend the Potomac, 150 years ago yesterday (September 23), Barnard again called upon the secretary about matter:

By letter of the 1st instant. I represented the importance of speedily arming the two works built for the defense of the Potomac approach to Washington. At your request I mentioned the number or improved sea-coast guns which I thought should be immediately supplied, and I mentioned eight, in consideration of the great demand for guns at the different sea-ports.

This was an off-hand statement, and I have since reflected on the matter, and have come to the conclusion that since there is no armament in Fort Washington of any value whatever, and that these two works will constitute, just now, the real defenses of Washington against maritime attack, the full armament of these works (namely, three 15-inch guns and thirteen 200-pounders) should be furnished very speedily. In case of war with a maritime power, allied with the rebellion, the defense of Washington can hardly be considered second in importance to that of New York.

I have, therefore, to request that in your directions to the Ordnance Department it may be directed to furnish the last-mentioned number of guns as speedily as possible.

Conventional wisdom is that all chances of foreign recognition, and thus intervention of a maritime power in support of the Confederacy, faded with the repulse of Pickett’s Charge.  Yet, here Barnard cites it as if a imminent threat.  Eventually Barnard would get those guns:

Now was the threat of a “maritime power,” allied with the Confederacy, with ironclads on the Potomac much of a real threat in the summer of 1863?  Or was Barnard trying to perfect the defenses while he had leverage?

Consider the quantities of heavy guns received by the Army after Gettysburg:

  • 8-inch Rodman Guns, 123 delivered  from a total of 213 produced.
  • 10-inch Rodman Guns, 1270 of 1301 total produced.
  • 15-inch Rodman Guns, 313 out of 323 produced.
  • 10-inch Parrott Rifles, 40 out of 42 produced.
  • 8-inch Parrott Rifles, 69 out of 91 produced for the Army.
  • 6.4-inch Parrott Rifles, 98 out of 233 produced for the Army.

The tallies don’t count for experimental types or those delivered to the Navy but borrowed by the Army.  But the numbers do include those delivered after the war, on wartime contracts.

Could we make the case that the Army capitalized on the increased wartime spending in order to “get healthy” on what was still considered the primary mission?  That mission being coastal defense, of course.

(Citations from OR, Series I, Volume 29, Part II, Serial 49, pages 149 and 226.)

Converted Rodman Rifles: Defending the coast in the time of lean budgets

Earlier this week, Keith Harris asked for a quick cannon identification via Twitter. Feel free to click over to Keith’s picture, but the gun below from Fort McHenry, Maryland is similar.

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8-inch Rifle Converted from a 10-inch Rodman at Fort McHenry

The exterior shape of this gun should look familiar. It’s a 10-inch Rodman. But a close look at the bore reveals something decidedly non-standard for that type of gun. This was a post-war modification to the gun. The story is interesting not only because it involves a Civil War cannon, but also because it carries forward all the way to the 20th century.

During and shortly after the Civil War the Army felt the Rodmans were sufficient to defend the coastlines. The combatants in the war deployed the most advanced warships of the day, and the Rodman guns rated well enough to deal with that threat. However within a decade, developments in Europe eclipsed the Civil War era weapons. Ship builders turned to steel, over the wood and wrought iron of the Civil War. So the coastline defenses needed improved guns.

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Pair of Converted 8-inch Rifles at Fort McHenry

Trouble was the U.S. had a lot of coastline to defend, but not a whole lot of money to spend on guns. Given the limited budget for updates, the Army looked around for options… and saw hundreds of Rodman smoothbores.

Recall that during the Civil War many of the old 32-pdr and 42-pdr guns got a new lease on life as “rifled guns.” With that experience in mind, soon ordnance officers and weapon developers stepped forward with different schemes to rifle those Rodmans. I’ve seen well over a dozen different plans for rifling Rodman guns, including plans for 15-inch conversions to 12-inch rifles. Some even provided for breech loading conversion.

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Muzzle showing breech insertion

The most successful (and I use that qualifier in relative terms) conversions involved the 10-inch Rodman guns. At least four different groups of Rodmans in that caliber underwent conversion to 8-inch rifles. Those four conversion plans required the original gun to be reamed out to fit new bore sleeve, and often a liner, by way of insertion. The sleeve and liner were usually steel, but some early experiments used wrought iron. The groups differed in the technique used for inserting the sleeve (through the muzzle or through the breech) and the number of rifle grooves.

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8-inch Converted Rifle on blocks at Fort McHenry

All told the Army contracted for conversion of over 200 of the 10-inch Rodmans from about 1876 to 1890. After successful tests at Sandy Point, New York, the guns went to the forts as the primary anti-ship weapon in the arsenal. However even at that time, everyone recognized the conversions were obsolete given the armament of European navies. But that was all the Army could acquire given the limited peacetime budgets.

These weapons remained in the Army’s inventory to the end of the century. The 1891 Manual of Heavy Artillery Service provided a range table and a dozen pages of drill instructions.

So 4700 yards range for a 35 pound powder charge. Still not all that impressive compared to contemporary breech-loading, steel rifles. For instance the “new” M1888 8-inch breechloading gun on a disappearing carriage rated over 10,000 yards. But of course the Army didn’t receive many of those “new” guns until the late 1890s.

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8-inch Rifle showing breech insertion plug

These converted rifles remained in the forts even during the Spanish-American War. And a good number of them survive today (as seen in the photos supporting this post). The service history of these guns spans four decades. Heck, these guns rival the B-52 bomber in terms of service life!

The gun Keith photographed is located in the Grand Army of the Republic plot of the Rosedale Cemetery in Los Angeles. I’m sure Keith will have a story to tell about that aspect of the gun’s history on his blog. To me, it looks like a gun which needs some tender loving care… and a few coats of paint.

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.

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  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.”

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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.

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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.