372 THE -SCIENTIFIC PAPERS OF 



influence upon its readings. The instrument would not indicate, 

 for instance, the existence of a local depression surrounded by 

 elevated ridges or plateaux, nor would it indicate the existence of 

 a peak. Considerable variations must therefore be occasionally 

 expected between the readings of this instrument, however cor- 

 rectly adjusted, and the results of actual soundings ; but it may 

 be observed that broken ground, such as would cause these 

 differences, is comparatively rare below the sea, which deepens 

 gradually from the land in such a way that the contour lines of 

 uniform depth can generally be distinctly traced ; and the 

 principal value of the instrument would consist in its indicating 

 its passage above varying depths. The indications of the instru- 

 ment must coincide very nearly with those of a sounding-line 

 upon an even slope, because the comparative proximity of the 

 ground towards the rise of the slope will be balanced by the 

 absence of solid matter towards its descent. 



Attention has already been called to Sir George Airy's observa- 

 tion of the greater apparent gravitation on islands than on the 

 sea- shore, and there than inland, and also to Professor Stokes's 

 explanation of the matter. The working zero of the bathometer 

 may be taken as a maximum or island indication ; and the 

 diminution due to the depth of water is therefore not influenced 

 by the irregularities met with on solid land, in consequence of the 

 matter raised above the natural surface of the sea. It has, how- 

 ever, been shown by -Archdeacon Pratt and others that continents 

 exercise an influence upon the level of the sea, that level being 

 raised up towards the masses piled above the surface ; and such 

 disturbance of the natural water-level must necessarily exercise an 

 influence upon the readings of the instrument. But this influence 

 would be perceptible only in estuaries or upon the sea-shore of a 

 mountainous continent, and may be neglected in dealing with the 

 surface of the sea under all ordinary circumstances. 



The more important disturbing cause affecting the instrument 

 under this head is that of the ellipsoidal form of the earth and the 

 varying centrifugal tendency on its surface, to which reference has 

 already been made. 



EFFECTS OF LATITUDE. The determinations of the effect of 

 latitude upon gravitation as made by Newton, Clairaut, M'Laurin, 

 and others have already been alluded to, and it is important that 



