TRANSACTIONS OF THE SECTIONS. 



203 



I 



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surface, it was found that the cork had been forced some distance along the tube, and 

 the amount of compression and of subsequent expansion were thus roughly estimated. 



The pressure gauge exhibited, may, in its present form, be considered as a small 

 hydraulic press ; of which the ram is forced into the 

 cylinder by the increasing pressure of the sea when 

 sinking, and expelled by the expansion of the water 

 in the cylinder when rising. It consists of a small 

 tube or cylinder having at one end a tap through 

 which water is admitted ; the tap having in addition. 

 to the opening admitting water, a smaller opening 

 for the escape of air. At the other end of the cylin- 

 der is a packing-box, through which a round bolt or 

 solid piston passes. A scale by the side of the pis- 

 ton contains the degrees of compression, and an 

 index at the further end of the scale is drawn along 

 the scale by the piston when forced by increasing 

 pressure into the cylinder, and secured by a spring 

 in its position, where it remains when the piston 

 is pushed back by expansion of water in the cylinder 

 to its former position. The scale and index are pro- 

 tected by a tube screwed on to the cylinder, and the 

 cylinder is protected from the risk of indentation by 

 an outer tube. In an experimental instrument the 

 packing-box has remained water-tight underthe appli- 

 cation of a pressure of 1000 lbs. to the square inch on 

 the piston ; so that the isolation may be considered 

 sufficiently perfect, as in actual use this pressure on 

 water in the cylinder would be counterbalanced by 

 the external pressure of the ocean. The packing-box 

 is just large enough to admit the packing, which con- 

 sists of vulcanized caoutchouc rings, stretched upon 

 the piston, and consequently adhering closely to it. 

 A moderate application of the packing- box screw 

 presses these rings against the packing-box, and a 

 perfect isolation of the water in the cylinder is thus 

 obtained. A slight lubrication of the rings, by the 

 addition of a small quantity of lard between them, 

 renders the amount of friction attending the motion 

 of the piston very trifling. 



In ascertaining the pressure of water, the amount . Cvlmder 



of friction overcome should be added to the com- g -p' * 



pression recorded by the index, to obtain the total C— Opening in Tap for admission 



amount of pressure. Some portion of the diminu- of water. 



tion of bulk will probably be occasioned by variation D — Opening in Tap for escape of 



of temperature, and which causes a greater variation air. 



in bulk at high temperature. As 4000 parts of sea- E — Packing-box. 



water at the temperature of 86° Fahrenheit, con- F— Packing-box Screw. 



tracted to 39S7 parts at the temperature of 65°, G— Piston. 



being rihi P arts f °r 21°; while from the tempera- **— Scale of compression. 



ture of 65° to 35°, the diminution to 3977 parts I— i<e S lster irulex - 



was only at the rate of 4 -i£ - parts for 30°, — the expansion and contraction of the 



cylinder by variation of temperature counteract the variation of water to a very small 



extent, being about 51 2 j ? th parts for 40° Fahrenheit. 



The experimental instrument indicates a compression of about one part in 20,000 

 per atmosphere (estimated at 15 lbs.) at the temperature of 60° Fahrenheit. 



The experiments will be varied by the use of a glass bulb with a long stem, finely 

 graduated, with a stopper of vulcanized caoutchouc, and in the tube an elastic ring 

 which is pushed during compression by the stopper towards the bulb, and remains 

 to mark the degree of compression. 



