meafuring Heights with the Barometer. 763 
whofe climate is fo very variable as that of Britain, fet- 
tled weather fhould be chofen as the belt time for obfer- 
vations. With any fudden fall of the barometer, in any 
affigned ftation below its mean height, it is apprehended 
that the rule will have a tendency to give defective re- 
fults ; and the contrary lhould happen when, from the 
increafed weight of the atmofphere, it rifes much above 
the mean height. 
From what hath been faid in the courfe of this paper, 
it will be perceived, that though the error of the rule is 
in general very fmall, yet now and then fuch irregulari- 
ties do occur as plainly fhew, that fomething ftill remains 
to be done, in order to perfect the theory of the baro- 
meter. 
The exigence, or otherwjfe, of a latitudinal equation 
being a point of the greateft confequence, lhould be de- 
termined with fo much care as to leave no doubt remain- 
ing on that head. And as this can only be effected by 
differences that are extremely obvious, the obfervations 
for that purpofe fhould be made at the equator, and as 
near as poffible to the poles'^. Peru is no doubt the beft 
lituation 
the heat of the air was founded, I have iince changed it to the mean barometer, 
or middle of the column of air intercepted between the two ftations. In this 
way all the great heights have been re-computed : the fmaller altitudes, not 
being fenfibly affe&ed by the alteration, continue as at firft. 
(t) Some idea may be formed what altitudes on the furface of the globe are 
accellible 
