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



and tuning-fork, replaced the smoked plate, and waited for the signal 

 as before. 



The operations were then repeated till half the prescribed shots 

 had been fired ; the gauges were then reversed on the piles, and the 

 same process again gone through. The greatest number of records 

 we ever succeeded in obtaining in a single day was sixteen. The 

 greater part of the time was taken up in adjusting the gauges and 

 preparing the charges. At the end of the day's work the gauges 

 were brought in, the firing wires wound up, and the plates packed in 

 a box for measurement at the University. Four complete records 

 were obtained on each plate. Shots were fired alternately from left 

 to right, and from right to left, past the piles and gauges, with a view 

 to the elimination of the time-constants of the recording apparatus. 



We have therefore the following experimental matters to ex- 

 plain : — 



1st. The measurement of the time of passage of the shock through 



water, from one gauge to the other. 

 2nd. The measurement of the distance between the gauges. 

 3rd. The precautions requisite to ensure symmetry of explosion, 



and of explosion in the line joining the two gauges. 



We shall consider the time-measurements first. In deciding on a 

 chronograph we had to bear in mind that it was an essential condition 

 of success to get an instrument which allowed of rapid and easy 

 manipulation. Since we required to make absolute measurements, 

 that is, to be able to reduce our chronograph indications to mean 

 solar seconds, it follows that some standard time measurer must be 

 adopted, even at the expense of otherwise reducing the handiness 

 of the apparatus, and of running the risk of a diminished sensitive- 

 ness. It is a truism that the methods which give the most absolute 

 values are seldom as sensitive as those which allow of a slight risk of 

 error in the absolute value of their indications. Considerations such 

 as these led to the preference being given to an instrument of the 

 falling pendulum or myograph description. Such an instrument was 

 therefore commenced in January, 1887, by Mr. Cook, the University 

 Assistant in the Physical Laboratory, though at the time his appliances 

 were of the most meagre description. The pendulum was about a 

 yard long, the bob consisting of a flat oval mass of lead. This carried 

 the supporting and adjusting screws for the glass plate. There were 

 holdfast catches for the bob at each end of its swing. On the base of 

 the instrument there were three separate appliances. First, a stand 

 to support the tuning-fork, and allow of its easy adjustment in a 

 vertical plane, and in a plane parallel to the plane of motion of the 

 pendulum. The plane of the latter was invariable, as it swung on 

 hard steel knife-edges, supported on shallow grooves cut in steel plates. 



