384 THE SCIENTIFIC PAPERS OF 



effect upon the instrument which might be expected, in consequence 

 of the combined influence of centrifugal force and ellipticity of the 

 earth. 



Criticisms have appeared in several papers questioning the 

 applicability of the bathometer for determining the depth of the 

 sea, owing to the disturbance of the sea-level by continental 

 attraction. This cause of disturbance had not escaped my atten- 

 tion in writing my paper * ; and it should be borne in mind that 

 the instrument cannot do more than indicate comparatively small 

 variations in total terrestrial attraction, which the hydrographer or 

 navigator using the bathometer will have to interpret according 

 to the circumstances of the case. The zero-point of the instrument 

 must vary no doubt with latitude, continental attraction, and also 

 in consequence of special geological causes ; but it is important to 

 observe that these causes are of a permanent character, and that if 

 an ocean has been once surveyed with the aid of the bathometer, 

 such special local conditions would become observed facts, and so far 

 from hindering the advantageous use of the instrument, would 

 serve, on the contrary, to increase its measure of usefulness in the 

 hands of the navigator. 



In the Addendum to my paper of the 23rd February, I described 

 a modification of the principle of the bathometer, designed for the 

 purpose of measuring horizontal attraction ; and I take this 

 opportunity of stating that I have constructed an instrument of 

 this description, which has been erected upon a solid foundation at 

 the Loan Exhibition, South Kensington. The measure of sensi- 

 tiveness of this instrument is given by the fact, that the weight of 

 a person stepping from one side of it to the other causes the 

 indicating bubble to travel through one division (of 1 millim.) of 

 the scale. It would not be difficult to construct such an instru- 

 ment of still greater sensitiveness ; and I believe that it could be 

 made a useful adjunct at physical observatories, for the observa- 

 tion of diurnal changes in the horizontal attraction produced by 

 the sun and moon as well as of terrestrial causes of disturbance of 

 the superficial equilibrium of the earth. 



* See page 372. 



