TRACT

Place of Greatest attraction.

71

.v — a X ( 1 — y' i) = '2929 a. Which shows that the otherlimit is ; that is, when the hill is extremely low, thepoint of greatest attraction is at of the altitude, like asit is at when the hill is very steep. And between theselimits it is always found, it being nearer to the one or theother of them, as the hill is flatter or steeper.

27. Thus then we find that at of the altitude, or verylittle more, is the best place for observation, to have thegreatest attraction, from a hill in the form of a triangularprism, of an indefinite length. But when its length is li-mited, the point of greatest attraction will descend a littlelower ; and the shorter the hill is, the lower will that point^.descend. For the same reason, all pyramidal hills havek their place of greatest attraction a little below that above^determined. But if the hill have a considerable space flatffilat, the top, after the manner of a frustum, then the saidipoint will be a little higher than as above, found. Com-Jmonly, however, t of the altitude may be used for the bestP place of observation, as the point of greatest attraction willseldom differ sensibly from that place. And when uncom-mon circumstances may produce a difference too great to-be intircly neglected, the observer must exercise his judg-ment in guessing at the necessary change he ought to makein the place of observation, so as to obtain the best effectp which the concomitant circumstances will admit of.