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



trumpet ends in a small tube, closed with an india-rubber film dia- 

 phragm. Against this diaphragm rests an independently supported 

 strip of aluminium foil, as fine and light as possible ; at its end it 

 carries a tiny point of platinum. The fine diaphragm, with its 

 tube and aluminium contact, is protected by an arrangement of flanges 

 carrying a glass cylinder, with stout brass ends. Through one brass 

 end passes a long brass screw, carrying a platinum disk at its lowest 

 point. This disk is insulated from the brass portion by means of 

 ebonite, and is connected by a wire with an insulated binding screw 

 outside the apparatus. The screw is adjusted so that it just clears 

 the platinum point on the aluminium spring. If the diaphragm is 

 disturbed the point is thrown against the disk and makes contact. In 

 order that the contact may be quite certain, the end of the platinum 

 wire must be cut off quite clean, and the platinum disk carefully 

 cleaned. The contact obtained by means of a platinum wire with a 

 fused end is useless, as its definition is more a matter of pressure than 

 anything else. As the apparatus has to be sunk so that its lower 

 end goes to a depth of 5 or 6 feet in water, it is of course necessary 

 to pump in air to distend the india-rubber, closing the mouth of the 

 trumpet, against the water-pressure. This implies the use of a simple 

 pressure gauge, and will also explain the necessity for the air-tight 

 glass chamber in front of the fine rubber diaphragm. In order that 

 the pressure on both sides of this diaphragm may be normally the 

 same, a "leak" is provided, running from the tube supporting the 

 diaphragm to the space in front of it. This " leak " consists of a bit 

 of drawn-out thermometer tubing, about an inch long. 



The apparatus with which all the earlier experiments were per- 

 formed is only suitable for explosions of from 6 or 7 oz. of gun- 

 cotton. In this apparatus the big trumpet is replaced by a drum 

 connected with the upper portion of the apparatus by means of a 

 brass tube having an internal diameter of nearly three-quarters of an 

 inch. The diameter of the drum was 6 inches, and each drum-head 

 consisted of an india-rubber diaphragm, so that each gauge had two 

 india-rubber faces. In order to prevent the india-rubber cutting at 

 the edge of the drum, the latter must be finely polished round the 

 rim, and this should, if possible, be made of round brass wire. We 

 found that the decay of the india-rubber was lessened by lubricating 

 the rim of the drum with tallow. By far the best, thing to use for 

 binding the india-rubber on to the drums is silk fishing-line. 



Both kinds of gauges were fixed to substantial wooden supports, 

 and these were provided with copper or iron cleats to run in guides 

 on a thick plank spiked on to the " turpentine " pile. The drum 

 gauges were sunk to the required distance by means of lead 

 sinkers ; the trumpet gauges required to be hauled under water by a 

 block and tackle attached to a 5-cwt. sinker dropped at the foot 



