ANIMAL MECHANICS. 



433 



amount of contraction n proper to each sphere being deter- 

 mined as a function of r by means of equation (no). 



We are familiar, in the construction of large guns, with 

 an instance that may be regarded as the converse of the sup- 

 posed case of the heart just given. A large piece of ordnance 

 cannot be made strong enough to resist the explosive force 

 of a heavy charge of powder, merely by increasing the thick- 

 ness of the gun ; for according to principles similar to those 

 employed above (using square roots instead of cube roots), it 

 can be shown that the inner rings of the gun are called upon 

 to do more than their share of the work of resistance, while 

 the outer rings are called upon to do less than their share. 

 Hence it follows that the inside portion of a very large gun 

 might be torn up by the force of the explosion before the 

 outside portion had suffered any strain at all. In order to 

 prevent this, and to compel every portion of the gun to 

 take its proper share of the work of resistance, large guns 

 have been made in separate rings, the outermost rings being 

 shrunk on while red hot, so that when cold the inner rings 

 are in a state of compression, and the outer rings are in a 

 state of tension. When the explosion now takes place, the 

 work done by the outer rings is represented by the compres- 

 sion of the inner rings, and these latter take no share in the 

 work of resistance until the force of the explosion has 

 destroyed their state of compression caused by the resistance 

 of the outer rings; and by this ingenious contrivance, an 

 attempt is made to compel every particle of metal of which 

 the gun is composed to bear its own individual fair share of 

 the work to be done. 



The celebrated Armstrong 600 pounder gun was formed 

 in this manner of eight distinct rings, and when the charges 

 of powder were gradually increased until the gun was burst, 

 it was found that the 1st, 6th, and 8th rings were burst, the 

 remaining five being apparently uninjured. If the principle 

 2 F 



