[Illustration:  FIG. 60.—­CRUSHER GAUGE. E, GAS CHECK.]

Hollow copper cylinders are manufactured with reduced sectional areas for measuring very small pressures.  It has been found that these copper cylinders are compressed to definite lengths for certain pressures with remarkable uniformity.  Thus a copper cylinder having a sectional area of 1/12 square inch, and originally 1/2 inch long, is crushed to a length of 0.42 inch by a pressure of 10 tons per square inch.  By subsequently applying a pressure of 12 tons per square inch the cylinder is reduced to a length of 0.393 inch.  Before using the cylinders, whether for experimenting with closed vessels or with guns, it is advisable to first crush them by a pressure a little under that expected in the experiment.  Captain Sir A. Noble used in his experiments a modification of Rodman’s gauge. (Ordnance Dept., U.S.A., 1861.)

By Calculation.—­To calculate the pressure developed by the explosion of dynamite in a bore-hole 3 centimetres in diameter, charged with 1 kilogramme of 75 per cent. dynamite, Messrs Vieille and Sarrau employ the following formula:—­

P = V_{o}(1 + Q/273._c_)/(V — v).

Where V_{o} = the volume (reduced to 0 deg. and 760 mm.) of the gases produced by a unit of weight of the explosive; Q the number of calories disengaged by a unit of weight of the explosive; c equals the specific heat at constant volume of the gases; V the volume in cubic centimetres of a unit of weight of the explosive; v the volume occupied by the inert materials of the explosive.  The volume of gas produced by the explosion of 1 kilogramme of nitro-glycerine (at 0 deg. and 760 mm.) is 467 litres.

V_{o} will therefore equal 0.75 x 467 = 350.25.

The specific heat c is, according to Sarrau, .220 (c); and according to Bunsen, 1 kilogramme of dynamite No. 1 disengages 1,290 (Q) calories.  The density of dynamite is equal to 1.5, therefore

V = 1/1.5 = .666.

If we take the volume of the kieselguhr as .1, we find from above formula that

P = 350(1 + 1290/(273 x .222))/(.600 — .1) = 13,900 atmospheres,

which is equal to 14,317 kilogrammes per square centimetre.  The pressure developed by 1 kilogramme of pure nitro-glycerine equals 18,533 atmospheres, equals 19,151 kilogrammes.  Applying this formula to gun-cotton, and taking after Berthelot, Q = 1075, and after Vieille and Sarrau, V_{o} = 671 litres, and c as .2314, and the density of the nitro-cellulose as 1.5, we have (V = O)

P = 671(1 + 1075/(273 x .2314))/.666 = 18,135 atmospheres.

To convert this into pressure of kilogrammes per square centimetre, it is necessary to multiply it by the weight of a column of mercury 0.760 m. high, and 1 square centimetre in section, which is equal to increasing it by 1/30.  It thus becomes

P^{k} = (1 + 1/30).

P^{k} = 18,135 x 1.033 = 18,733 kilogrammes.