510 Prof. R. Threlfall and Mr. J. F. Adair. On the 



wards succeeded in making it, and consequently there is every reason 

 to believe that the difference in time- constant observed in these 

 experiments was very much greater than it was in the actual observa- 

 tions. But the times measured are of the order of one-tenth of a 

 second, and the amount we are now concerned with is of the order of 

 O0006 second, so it is clear we are discussing the correction of a 

 correction. 



It is true, however, that the instantaneous pressures to which the 

 gauges are exposed during explosions differ by considerably greater 

 amounts than those due to the action of the two pendulums ; it was 

 always arranged, therefore, that the gauges should have a considerable 

 excess of sensitiveness when used to measure the explosion velocities. 

 It must be added that the length of the chord of the arc through 

 which the pendulums swung in. the testing experiments was about 

 20 cm., so that the whole time in question is only of the order of the 

 time taken by the pendulums to swing through about half a millimetre 

 at their greatest velocity. In addition to this, it was extremely 

 difficult to keep the tinfoil properly and uniformly glued to the 

 surface of the rubber. The effect of the blow was always to crumple 

 the tinfoil to a small extent ; a wrinkle of the order of O'l mm. would 

 explain the whole effect, and after several impacts much greater 

 deviations were difficult to avoid. The blows from the lead bob 

 resulting in by far the greatest deformation of the rubber produced 

 much the greatest trouble in wrinkling the tinfoil, and yet we could 

 not replace the tinfoil after such experiment because we desired to be 

 sure that the state of the surface as to greasiness was constant, and 

 consequently we were obliged to smooth the tinfoil down to the best 

 of our ability. It may be added that, of course, the tinfoil was as 

 well cleaned as possible, but it is not a very easy substance to be 

 quite sure about in this respect, and the thinnest silver and platinum 

 foil we had and tried would not stick to the rubber properly unless 

 stuck by so much marine glue as to weight the diaphragms sensibly, 

 and thus perhaps introduce a peculiarity. Having in view the 

 sources of error above indicated, we did not feel justified in applying 

 any correction to the observed velocities of explosion-effect on this 

 account ; the effect, therefore, will be that our velocities run the risk 

 of being a little too high to the extreme extent of, say, 2 per cent. ; 

 we are certain ourselves, however, that this is not the case. It may 

 be mentioned that the experiments with the rod pendulums led, on 

 the whole, to rather smaller values for the time-constants, but we 

 have not laid any stress on them, as the pendulums did not move 

 with exactly equal velocities in all cases. 



