Velocity of Transmission of Disturbances through Sea-water. 503 



of the pile. The handling of these big trumpets required several 

 men ; even the smaller gauges were most troublesome to fix if the 

 weather was bad. The chief difficulty met with in the earlier forms 

 of gauge was their liability to leak ; in the later forms this was got 

 over by the method of tying on the india-rubber and by cementing 

 in the glass cylinder between two concentric rings of brass with 

 melted sealing-wax. (xlue mixed with bichromate of potash and 

 melted in the dark is also an excellent cement;, if coated with shellac 

 varnish. The screw carrying the platinum disk passed through a 

 stuffing box filled with an india-rubber bung, which could be jammed 

 tight by means of a screw top. During the course of the experiments 

 it was constantly necessary to adjust the depth of the gauges to allow 

 for the rise and fall of the tide. 



In the experiments, the results of which are given in this paper, 

 the " drum " gauges were exclusively employed. This is due to two 

 causes. In the first place, we seldom commanded sufficient manual 

 assistance to work the trumpets properly ; and in the second, these 

 latter were so sensitive to the action of small waves on the surface of 

 the water that we could not utilise their full sensitiveness for the 

 purpose of registering the explosions. Except on some few days in 

 the course of the winter, the requisite calmness of the water is not 

 attained in Port Jackson. It may be mentioned that it is easily 

 possible to set these large gauges to such a point of sensitiveness that 

 they will make contact on receiving the verbal order to do so — if it is 

 uttered in the right tone from a distance of 10 or 15 yards. 



Appliances connected with the Firing of the Charges. 



The charges themselves were always enclosed in thin tin cylinders, 

 or else in oil-silk or mackintosh. Except with respect to the detona- 

 tors, there is little to say. When we were driven to make them for 

 ourselves, we finally, with the assistance of Mr. Cook, hit on the 

 following method. A wooden mould was prepared, of such a form 

 that it allowed a cylinder of plaster of Paris to be cast round two 

 lengths of gutta-percha-covered copper wire running parallel to each 

 other and to the long axis of the cylinder. When the plaster had 

 set, the wires were cut off at one end to within about half a millimetre 

 of the end surface of the cylinder, and at the other were allowed to 

 protrude to a distance of 5 or 6 inches. The fusible bridges of 

 thin platinum wire were secured to the short ends of the copper wire 

 by solder and resin only. These bridges were primed with a good 

 covering of a mixture of guncotton fluff and meal powder, made into 

 a paste with collodion. In order to attach the detonating cap, advan- 

 tage was taken of the good workmanship of ordinary cartridge cases. 

 A number twelve bore " case was taken and cut off above the brass 



