VOL. LXV.] ' PHILOSOPHICAL TRANSACTIONS. 701 



traction, which cannot be placed far from the middle of the mountain. Hence 

 the plumb-line of a quadrant, or any other astronomical instrument, must be 

 deflected from its proper situation, by a small quantity towards the mountain ; 

 and the apparent altitudes of the stars, taken with the instrument, will be altered 

 accordingly. >n'uton't 



It will easily be acknowledged, that to find a sensible attraction of any hill, 

 from undoubted experiment, would be a matter of no small curiosity, would 

 greatly illustrate the general theory of gravity, and would make the universal 

 gravitation of matter as it were palpable, to every person, and fit to convince 

 those who will yield their ascent to nothing but downright ex{)eriment. Nor 

 would its uses end here, as it would serve to give us a better idea of the total 

 mass of the earth, and the proportional density of the matter near the surface, 

 compared with the mean density of the whole earth. The result of such an un- 

 common experiment, which I should hope would prove successful, would doubt- 

 less do honour to the nation where it was made, and the society which exe- 

 cuted it. 



Sir Isaac Newton gives us the first hint of such an attempt, in his popular 

 Treatise of the System of the World, where he remarks, " That a mountain of 

 an hemispherical figure, 3 miles high and 6 broad, will not, by its attraction, 

 draw the plumb-line 2 minutes out of the perpendicular." It will appear, by a 

 very easy calculation, that such a mountain would attract the plumb-line l' 18" 

 from the perpendicular. ..ci 



But the first attempt of this kind was made by the French academicians, who 

 measured 3 degrees of the meridian near Quito in Peru, and who endeavoured 

 to find the effect of the attraction of Chimboraqo, a mountain in that neigh- 

 bourhood, which is elevated near 4 miles above the sea, though only about 2 

 miles above the general level of the province of Quito. By their observations 

 of the altitudes of fixed stars, taken with a quadrant of 2-*- feet radius, they 

 found the quantity of 8" in favour of the attraction of the mountain, by a mean 

 of their observations. This indeed was much less than they expected; but then 

 it is to be considered, that their instrument was too small and imperfect for the 

 purpose; and that they themselves were subject to great inconveniencies, being 

 sheltered from the wind and weather by nothing but a common tent, and placed 

 so high up the mountain as the boundary where the snow begins to lie unmelted 

 all the year round. And indeed their observations, doubtless owing to these 

 causes of error, differ greatly from each other, and are therefore insufficient to 

 prove the reality of an attraction of the mountain Chimboraqo, though the 

 general result from them is in favour of it. Accordingly, one of the French 

 gentlemen themselves, M. Bouguer, who drew up the account of their experi- 

 ment, expresses his wishes, that a like experiment might be made, to find the 



