W. E. Woodbridge on the Pressure of Fired Gunpowder. 155 



which the pressure was restrained, a piezometer* which by regis- 

 tering the compression of the oil which it contained, should 

 indicate the pressure to which it was exposed. The piezometer 

 used in the experiments is a small cylindrical vessel of steel, 

 inclosing a quantity of oil which receives the pressure of the 

 fluid by which it may be surrounded through the medium of a 

 piston which will move inward a distance proportioned to the 

 amount of compression. To the piston is attached a stem of 

 wire, projecting inward, and receiving on its side the pressure of 

 a fine point, which, when the piston is moved, makes a line on 

 the stem, equal in length to the distance through which the pis- 

 ton moves. In order that the mark may be more distinctly visi- 

 ble, the stem is coated with a thin film of black varnish. A 

 partial rotation of the piston, after the adjustment of the quantity 

 of oil, inscribes a transverse line on the stem, from which to 

 measure the one denoting the compression. The length of the 

 mark is measured under the microscope by means of a rule divi- 

 ded into T¥ Vo tQS °f an inch. The details of the construction of 

 the piezometer are arranged with reference to obtaining as great 

 capacity and as great length of stroke as its exterior dimensions 

 would permit — to fixing the proper relation between the area of 

 the piston, the capacity of the instrument and the pressures to 

 which it was to be subjected, and to its being easily filled with 

 oil, and the quantity adjusted without including air. 



The experiments on the compressibility of oil necessary to 

 determine the pressure per square inch corresponding with a 

 given length of stroke, at a given temperature, were carefully 

 made. The amount of compression was subject to actual inspec- 

 tion, up to pressures of 10,000 lbs. per square inch. The oil 

 submitted to trial was enclosed in an instrument of glass consist- 

 ing of a bulb and graduated tube. The scale upon the tube was 

 marked by means of a dividing-machine, and the capacities of 

 its divisions were equal, so far as determined by a careful exam- 

 ination with columns of mercury, of different lengths. The 

 capacity of each division was equal to one part in 3762*2 of the 

 volume of the oil at 60° Fahr. To the bore of the tube (0"038 

 in., in diameter) was fitted an iron piston, packed by a ring of 

 mercury occupying a groove turned in its edge. This arrange- 

 ment was found to favor accurate observation, and to answer its 

 purpose well in all respects. The instrument was enclosed in a 

 strong tubular receiver, having windows of glass, through which 

 it could be inspected. The windows are truncated cones, having 

 their bases inward, and are fitted to conical cavities in opposite 

 sides of the receiver. A rack and pinion, worked by a little 

 shaft passing through the side of the receiver, serve to bring all 



* The use of the -word piezometer to denote an instrument for the measurement of 

 pressure instead of compression, certainly accords with its derivation. 



