856 Ordnance Survey Astronomical Observations. 



Now this abnormal attraction can be only that of gravitation, 

 which depends solely on weight ; as, for instance, a pound of 

 gold, a pound of stone, and a pound of ice. placed at equal 

 distances from each other, will act each on the other with 

 precisely similar gravitating forces. If therefore the weight 

 and distance of a mountain be known, its effect on the plumb- 

 line can be computed ; and the first question is, whether the 

 observed deviations can be completely explained by such visible 

 and tangible excrescences on the surface of the earth, or 

 whether they be owing to some heterogeneous construction 

 beneath it? If the former be the case, the elimination is 

 comparatively easy but laborious, for the mountain can in 

 time be measured ; there it is ; but in the latter case, where 

 is it, i.e., the attracting body, or bodies V 



At one of their trigonometrical points in Peru, the savants 

 of Louis XIV. found an attraction in the mass of the Andes 

 to the amount of 7" : and more recently Mr Maclear found 

 the attraction of Table Mountain equal to 3", and that of 

 Piket Berg equal to 2", at the stations employed by a former 

 measurer of an arc of the meridian in South Africa. In his 

 own determinations therefore, Mr Maclear left the sheltered 

 valleys around the foot of a mountain, and preferred all 

 the inconveniences of placing his instrument on the exposed 

 summit : for there, having the mountain beneath his feet, 

 its attraction acted, — in so far as its mass was uniformly 

 disposed about its culminating point, — in the direction of that 

 of the earth, and therefore could produce no deviation from 

 the vertical. 



But in England and Scotland, there are no mountain ranges 

 like the Andes ; nay, in the neighbourhood of the survey ter- 

 minal stations, there are no masses comparable to TableMoun- 

 tain or to Piket Berg ; and yet the effects of local attraction 

 at many of the stations are greater than were observed in 

 the actual neighbourhood of those giant ranges. The known 

 configuration of the surface of the dry land therefore will not 

 explain completely all the difficulties, nor will any probable 

 shape that may be given to the land under the neighbouring 

 seas. 



Magnetic and electric affinities, it need hardly be observed, 



