.528 



REPORT — 1894. 



is screwed a piece of copper, ii. The pressure of the gas acting on the 

 piston I forces the knife into the copper ; by mechanical means a similar 

 cut can be produced, and hence the magnitude of the cut gives the measure 

 of the pressure which has produced it. A small cup at c prevents any 

 gas passing the indenting tool. 



The great improvements that Major Rodman made in gunpowder are 

 well known. To him we are indebted both for the earliest experiments 

 on the effect of the size of grain on tlie maximum pressure and for the 

 powder adopted by all nations for large guns, I mean prismatic powder ; 

 but it is a question whether he was not in some degree led to these great 

 improvements by an erroneous estimate of the pressures produced, this 

 erroneous estimate being mainly due to the necessity of placing the 

 Rodman gauge at the exterior of the gun ; and the efl'ect of this objec- 

 tionable position would be greatly exaggerated if the powder experimented 

 with were of a brisante nature. 



It is curious that so distinguished an artillerist as Major Rodman 



Fig. 1.— Eodman's Pressure Apparatus. 



should never have taken the trouble to calculate what energies the 

 pressures which his instrument gave would have generated in a pro- 

 iectile ; had he done so he would have found that many of the results 

 indicated by his instrument were not only improbable but were absolutely 

 impossible. 



As an illustration of Major Rodman's method I take an interesting 

 series of experiments made in smooth-bored guns of 7-inch, 9-inch, and 

 1 1 -inch calibres, and so arranged that in each gun an equal column or 

 weight per square inch of powder was behind an equal column or weight 

 per square inch of projectile. Under these conditions, in each gun, during 

 the passage of the shot along the bore, the gases would be equally ex- 

 panded, and the energy per unit of column developed at every point in 

 the three guns should be the same, except for slight differences on account 

 of increased temperature and pressure in the larger guns, due to the 

 smaller cooling surface in proportion to the weight of charge. 



Major Rodman measured his pressures at the base of the bore and at 



