Captain Blakely wrote a long letter to his counterpart Captain John Dahlgren of the United States Navy during March 1858, outlining his progress in ordnance to that date. He gives his calculations and reasoning for his principles in considerable detail. Captain Dahlgren’s response to this letter though sent has been lost.

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                                                                 Army & Navy Club, St James’s Sqr

London

   March 19, 1858

Sir

I trust you will pardon the liberty I take in addressing you on a subject with which you are probably more familiar than myself. Still, the results of experiments I have been making may be interesting to you, and should they convince you of the truth of the new theory I advocate, as they have even the officials here, I foresee that you will be able to inaugurate a new era in naval warfare by using shell guns of much larger than are now made.

You are aware that in a large gun the pressure of the gas of the powder is greater than in a small gun, because the  powder, burning in a proportional time; the shot does not move so as to leave a similarly proportional space for it to expand in.

From some experiments I have discovered the law that the pressure in an old musket never exceeds 300 atmospheres, which in an 18″ gun it must for an instant reach 3000. In a 32 pounder when 2 solid shot are fired, it probably reaches 1500 atmospheres, the maximum cast iron can bear even for an instant – so that 6 inches is the largest size projectile for a cast iron gun to fire 2 solid shot with safety. Eleven inches will I think be found the maximum for shell guns of great calibre with a moderate charge of powder. There I understand guns you make about 12 inches thick at the breach [sic].

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I think that by casting the iron with a maximum external diameter of 27 inches – when turned – and cylindrical to the trunnions that a very much greater strength would be obtained by casting the part outside of this as a separate piece and boring it out to a cylindrical inner surface of 26 40/50 inches diameter. This to be heated and then allowed to shrink on to the inner mass, would thus be able to exert much more of its force than if merely added in casting. My reasons for thinking so are as follows. When the gun is fired the pressure of the powder causes the metal to extend the inner diameter, probably becoming 11 + 1/100 having originally been exactly 11.

The metal 8 inches from the bore (or where the diameter of the imaginary layer is 27 inches) cannot extend as such as it would become 27 + 27/1100, the cross section, or difference between the squares of these measurements in round inches would be much more than the cross section originally or 27² – 11² = 808 round inches. The cross section remaining the same we find that the metal at 8 inches from the bore can only do one-sixth as much work as the inner layer, because it can only extend 1/6 as much, or to 27 + 27/6000, in which case the cross-section remains (27 + 9/2200) ² – (11 + 1/100) ² = 808 round inches the same as before.

But as the metal, to exert its full strength must stretch 27/1100 the only way to make it do so is to have it originally 27/1100 – 27/6600 or about 1/50 inch less in

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diameter, so that when the strain comes it is stretching 1/50 + the push from inside, which we have seen to be only 27 / 6600, the #### being that desired – of course wrought iron will be preferable for the outer part and the thinner & smaller the plies [i.e. layers] the better.

All this you will find discussed by Professor Treadwell in a paper read to the American Academy around 1856 and published by Messrs Marshall & Co., Cambridge, U.S. Should you think well of the plan it is to him that the United States Government & people owe gratitude, as having first published the idea there after I patented it here on Feby 27 1855. In June of that year Mr Mallet read a paper to the Royal Irish Academy recommending the same views, but he has unfortunately tried to prove too much with his 36 inch mortars. These, though ten times as strong as if of cast iron, are yet not strong enough.

I have been more successful, and have succeeded in convincing the Woolwich Committee that for all sizes over 8″ superimposed plies must be used. I got a 9 pounder gun and turned it down cylindrical from the trunnion, the outer diameter being then 10 inches. On this I shrunk wrought-iron rings exactly replacing the weight removed but with metal capable of exerting nearly its whole force instead of only one-sixth of it. The inner diameter of the rings was 9.99 inches. This gun was tried against a cast iron gun the same size and weight & manufactured of the same iron by

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the same government contractor, at the same time and against brass service guns of the same calibre and against one made by Mr Dundas of Dundee of staves of wrought iron hooped together. The comparative trial was conducted by the Government people at Shoeburyness.

The guns were fired as follows

2 rounds with 8 lbs of powder 2 shot 1 wad

86 rounds with 3 lbs of powder 1 shot 1 wad

26 rounds with 4 lbs of powder 1 shot 1 wad

5 rounds with 3 lbs of powder 1 shot 1 wad

5 rounds with 6 lbs of powder 1 shot 1 wad

3 rounds with 6 lbs of powder 2 shot 1 wad

Here Mr Dundas’ gun burst. The remainder were fired 107 rounds with the last named charge when the cast iron gun burst. The brass gun and mine were fire 64 additional rounds with the same charge, when the brass gun became unserviceable. Mine was then fired 134 rounds more with the same charge (6 pounds of powder & 2 shot) but showed no signs of wear, except having required a new copper vent. 

The charge was increased by one shot at a time till the gun was loaded to the muzzle, in which state it was fired 158 times, when it burst. Such a triumphant result has produced conviction here, I take the liberty of communicating it to you for the sake of the advancement of science, hoping that it will enable you to use 20 inch guns at least. Should these calculations be of any service to you, your gratitude is due as I said before to Professor Treadwell of Cambridge, U.S., who first published in America, as I believe, I did first in Europe.

I remain Sir,

Yours faithfully

A T Blakeley [sic], Captⁿ, R.A.

To Captⁿ Dahlgren

U.S. Navy

Note:

The signature  on the original is NOT Blakely’s. Apart from the clearly inaccurate spelling the writing style is different from his known signatures. It may have been drafted or dictated by him and written out formally by a clerk, as was common at the time, as part of his publicity campaign.

Captain Blakely's signature as found on letters, invoices and receipts

The original document was discovered and scanned by

John Morris

of the