PROCEEDINGS OP THE POLYTECHNIC ASSOCIATION. 



513 



assuming the absolute zero at — 274° C, which is the latest determination as 

 given by Rankine. Putting then p for the pressure we shall have 



p = 379.58/ I -4- 3340 \= 5006-5 atmospheres. 

 \ ^74"/ 



This yalue exceeds that found by the experimenters themselves by 632.9 

 atmospheres. The difference is owing almowt entirely to the difference in 

 the assumed specific gravity of the powder, the experimenters having 

 taken this at .964, while we have employed the value 1.039. When gun- 

 powder is not shaken down, its specific gravity is always less than one 

 and when well shaken is always more than one. The U. S. Ordnance Manual 

 (edition of 1850), gives .929 for the specific gravity of loose powder, and 

 1.039 for that of powder well shaken down. It is obvious that, in a gun, 

 we must adopt the higher value. There is also a slight difference between 

 the determinations, owing to a difference in the assumed place of the abso- 

 lute zero. The co-efficient of expansion employed by the experimenters is 

 (1 + 0.003660t,) which corresponds to a zero at — 273'*.225 C. Adopting their 

 specific gravity with the zero at — 274*^, the pressure would be 4364, or about 

 ten atmospheres less than the determination of the experimenters. 



We are now in condition to apply the formula above given to the com- 

 putation of the velocities which the initial pressure just assigned ought 

 to be capable of generating in projectiles of given weight, fired with given 

 charges of powder from the guns of given calibre and length; and also the 

 initial pressures which would be necessary, in similar cases, to produce 

 the velocities actually observed. The examples which follow, twenty-five 

 in number, are taken from the U. S. Ordnance Manual, and exhibit the re- 

 sults actually obtained in experimental firing afr the Washington navy 

 yard. As the guns used were all smooth bores, and the projectiles round 

 shot, the observed velocity is corrected for the loss by windage. The 

 formula for this correction, which experiment has suggested, is 



c— b 



in which C is the correction, c and b have the values assigned them in the 

 foregoing formula, and A is a constant determined by observation, and is 

 usually put =6 400 ft. 



The particulars which enter into the calculation for each form of gun 

 are the following: 



In the tables which succeed are given the values of v, which result 

 from the formula when x is made equal to 1 — 1 -|- '•^', that is when it has its 

 value, which belongs to it at the moment the shot leaves the muzzle. 

 [Am. Inst.] H* 



