THE BATHOMETER. 131 



closed by means of a disc of plate glass, the variations in 

 density do not affect the quantity of air enclosed within its 

 casing, aud we thus get rid entirely of the disturbing 

 effects which would arise from this cause. 



At the bottom of the figure is shown an apparatus by 

 which I used to take the readings of the instrument, which 

 consisted of a micrometer screw to which an electric 

 contact piece was attached, and in screwing this micrometer 

 screw until contact was obtained, which was evidenced by 

 the ringing of a small electric bell, I could ascertain the 

 exact amount by which the diaphragm was being depressed 

 or raised by the natural balance of the two forces the 



elastic force and the gravity force ; but this mode of read- 

 ing was inconvenient, it required great care on the part of 

 the observer and did not furnish a ready indication of 

 the state of gravitation or of the depth of the sea below 

 the instrument without first going through this operation 

 of making the contact. The arrangement which I have 

 now adopted is of a much simpler kind. The mercury 

 ascends into the cup up to nearly its upper surface, the 

 remaining space is filled up by water, and the water ascend- 

 ing into the spiral tube upon the scale carries with it an 

 air-bubble, and it is the position of the air-bubble in the tube 

 which gives me the indications of the state of the instru- 

 ment. It will be readily seen that the amount of depression 

 in the cup will cause a proportionate motion of the bubble 

 in the cylindrical tube, and hence the amount of motion of 



K 2 



