28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 71 



safely be assumed that if the first upward rush does not tear the 

 paper, the force due to rebound that acts upon the gun must be small 

 compared with the impulse produced by the explosion of the powder. 

 It should be noted that the tissue paper tells nothing as to whether 

 or not there are a number of successive reflections or rebounds grad- 

 ually decreasing in magnitude ; neither does it give information con- 

 cerning the downward pressure the gases exert upon the chamber 

 tending to decrease the displacement, after they have accumulated in 

 the space between the top o'f the chamber and the cap, C, plate 6, 

 figure 2. 



DIRECT-LIFT IMPULSE-METER 



A section of the direct-lift impulse-meter is shown in figure 5(b). 

 It is also shown in the photograph plate 6, figure 2, at A. A small 

 cylinder A of aluminium of 1.46 grams mass, hollowed at one end 

 for lightness, was turned down to slide easily in a glass tube G. This 

 tube, G, was fastened by de Khotinsky cement to an iron wire W, 

 which was in turn fastened to the yoke Y, plate 7, figure i , so that the 

 glass tube, G, was held in a vertical position, between the chamber 

 and the wall o'f the 3-inch pipe — similarly to the tissue paper. Two 

 small wires C, C, of spring brass were cemented to the top of the 

 aluminium cylinder, the free ends just touching on opposite sides of 

 the glass tube. The inside of the glass tube .was smoked with cam- 

 phor smoke above the point marked X, so that a record was made of 

 any upward displacement of the aluminium cylinder. The cylinder 

 was prevented from dro'pping out of the glass tube by a fine steel 

 wire, w, cemented to the tube and extending across the lower end. 



The theory of the direct-lift impulse-meter is given in Appendix C, 

 page 61. From the theory, we may derive an expression for the 

 ratio, Q, of the momentum given the gun by the gaseous rebound, to 

 the observed momentum of the suspended system. 



There are two disadvantages of this form of impulse-meter. First, 

 friction acts unavoidably to reduce the displacement. Secondly, any 

 jar to which the apparatus is subjected on firing will cause the 

 aluminium cylinder to jump, and thus give a spurious displacement. 

 This latter fact rendered the meter useless for experiments in which 

 direct lift of the chamber took place, as there was always much jar 

 when the heavy chamber fell back, after being displaced upward. 



This impulse-meter, it will be observed, gave a mean measurement 

 of any successive up-and-down rushes of gas. 



