364 THE SCIENTIFIC PAPERS OF 



1859 on board H.M.S. " Firebrand," commanded by Capt. Day- 

 man, during a trip undertaken for the Admiralty for the purpose 

 of determining a line of soundings across the Bay of Biscay, with 

 a view to the establishment of a submarine cable : it proved 

 successful to the extent that I was able to predict, approximately, 

 the depth that would be found on the use of the sounding-line. 

 The difficulty, however, of observing the instrument was great, 

 owing to the excessive pumping-action, the consequence of the 

 oscillations of the ship, as well as to the difficulty of obtaining 

 perfect uniformity of temperature. The method of observation 

 pursued was to take series of ten observations of alternate maxima 

 and minima positions of the film, or boundary line between the 

 liquids, of which the mean was taken to be its true position upon 

 the instrument ; but occasionally oscillations of extraordinary 

 amount occurred, tending to vitiate the value of even these means. 

 The instrument was both bulky and delicate, and it was found 

 impracticable at the time to provide the ship with a sufficient 

 store of ice (to be used in maintaining the instrument at a uniform 

 temperature) to last during a lengthy voyage. In consequence of 

 these drawbacks, I relinquished for a time the idea of constructing 

 a reliable bathometer. 



PRESENT CONSTRUCTION OF BATHOMETER. Last year the prac- 

 tical difficulties encountered in laying submarine cables in water 

 the depth of which had not been accurately ascertained before- 

 hand, revived in me the conviction that an accurate instrument 

 would be of considerable value, not only to the cable -layer but to 

 the navigator generally, when unable to determine his position 

 astronomically. In the instrument about to be described, the 

 mercury column is retained as the representative of the force of 

 gravitation, but the balancing force is obtained through two spiral 

 springs, which are so adjusted to the force of the mercury column 

 that changes of temperature are entirely eliminated from the 

 result. 



The instrument, which is represented on Plates 31, 32, consists 

 of a tube of steel, with cup-like extensions at the two extremities, 

 which is suspended in a vertical position from a universal joint, 

 at some little distance above the centre of gravity of the system, 

 with a view of preventing pendulous action. 



The upper cup-like extension of the tube is closed with a lid, 



