THE BATHOMETER. 123 



the bottom and will be incapable therefore of following the 

 sinuosities of the ground. This proposition is proved 

 mathematically, that the retaining force must be equal to the 

 weight of cable. But how are we to ascertain the depth of 

 the sea so as to prevent undue loss of cable ? It might be 

 said that the sea should be sounded beforehand^, but in 

 laying a cable across the sea, say across the Atlantic, it is 

 not always easy to know where the currents interfere with 

 the path of the ship through the water, and if the weather 

 is at all unfavourable no astronomical observations can be 

 taken, and the actual position of the ship may be very 

 different from the assumed one, and consequently the actual 

 depth, however carefully the sea may have been sounded, 

 may be very different from what it is supposed to be. 

 Hence the difficulty and the importance of knowing the 

 depth of the water. It occurred to me whether it was not 

 possible to keep a running record of ithe depths of the sea 

 below the ship while the ship was moving. Of course a 

 sounding-line was out of the question, because it takes 

 several hours to let a lead sink down to the bottom of a 

 deep sea ; but was it not possible to take advantage of the 

 inferior specific gravity of the water itself, to gain an indi- 

 cation of the depth] TMs thought occupied my mind, 

 and I constructed an instrument which I thought would 

 give me an indication of the variations in the total gravi- 

 tation of the earth as the ship passed over the surface. 

 This instrument, however, was not perfect. I had to deal 

 with extremely slight variations in*the total attraction, and 

 an instrument to indicate such slight variations, notwith- 

 standing the disturbing influences of the motion of the 

 ship, variations of temperature and variations in the 

 pressure of the atmospheric column, was a problem of some 

 difficulty ; and after having tried an instrument and ob- 

 tained certain indications of success I abandoned the matter 

 until recent events (the occasion of laying the cable across 

 the Atlantic) recalled to my mind the great practical impor- 

 tance which such an instrument would be, not only for the 

 cable layer, but for the navigator generally. But before 

 we can consider the construction of such an instrument we 

 shall have to look to the general question involved. 



The law of gravitation, as you all know, was first deve- 

 loped by Newton. Newton's mode of viewing the question I 



