448 MEMOIR OF DANIEL TREADWELL. 



was first pointed out by Professor Barlow, some t)iirty years ago.* In the cannon as constructed 

 bv me, I have provided a remedy for this condition, by which I produce as near as iiossible the 

 result that all the substance of which the cannon is composed shall be eOjUally strained by the 

 explosive effect of the powder, and that no portion shall yield and rupture before every other 

 portion has received the greatest strain that it is capable of bearing, that is, when the force is 

 carried to the extent of producing rupture." 



In making these lioops they are heated, and by forging brought to their proper 

 dimensions ; in this process they necessarily become annealed, and consequently 

 inelastic. They must regain their elasticity and hardness to fit them for the purpose 

 intended. This, as is well known, can be brought about by subjecting them to ham- 

 mering or rolling. To ascertain the temperature at which this elasticity and hardness 

 cease, an elaborate series of experiments was made with iron wire of different sizes, 

 heated from 400° up to the temperature at which it is thorouglily annealed and its 

 elasticity lost, that is, to a full red heat. 



" These experiments demonstrate with some degree of precision several physical facts, all of 

 whicli arc of high importance in the construction of cannon upon tlie principle pointed out in tlie 

 JVlemoir to which this is a sequel. t Tlu^sc facts are: — 



^^ First, That witli a piece of iron hardened by compression and tension, in the condition of 

 hard wire, tlie amount of jiermauent elongation is far smaller than tlio permanent elasticity up 

 to near the breaking point, and also tliat the permanent elongation does not begin until about 

 one half of the breaking strain is applied. 



" Second, Tiiat the part of the elongation, or stretch, which is within the elastic power of the 

 wire, increases very regularly under equal increments of strain ; thus exhibiting the truth of 

 the maxim, Ut tensio, sic vis, — As the stretch, so the strain. But the permanent elongations 

 made by the same increments of strain, especially when near the breaking of the wire, are 

 entirely at variance with this maxim. This was shown in the experiment, where an increment 

 of 20 pounds to an existing strain of 120 pounds produces a permanent stretch of -/gths of an 

 inch, while the same increment of 20 pounds, when the wire was under a strain of 2a0 pounds, 

 increased the length, permanently, full IJ inches. 



" Third, Tliat, when the material has been subjected to a strain of a given amount (say 440 

 pounds), the repeated application of a strain within that amount produces no further permanent 

 elongation. 



" Fourth, That the subjecting of the same material to a heat sufficient to burn oil in contact 

 with it (supposed in this case to be 800° Fah., at least) will not impair its elasticity. 



'■'■Fifth, Tliat, when the iron is annealed, tlie permanent elongation commences at a compar- 

 atively low strain, and that its extent is very large in proportion to the elasticity of tiie iron, 

 which shows how inappropriate is the use, upon a cast-iron body, of a hoop that has been heated 

 to an annealing temperature ; as it must be loosened, or suffer the cast-iron to break within its 

 grasp, before a strain upon it up to half its tensile strength shall be reached." 



"To construct one of the lioops for a cannon of the size before mentioned, that is, of 14-inch 

 calibre, the hoop having, when finished, 27.972 inches' internal diameter, and being 3i inches 



* The condition limits the size of guns made entirely of cast iron, as is seen in the Rodman gun, and led Mr. 

 Treadwcll to malie the thickness of the walls of the cast-iron body of his gun only half the diameter of the bore, 

 t Memoirs of the American Academy, Vol. IX., New Series, 18C4. 



