meafuring Heights with the Barometer. 761 
contider as very irregular, and do not pretend to account 
for, is that for the height of Moel Eilio, a hill fituated 
between Carnarvon and Snowdon: the real altitude 
2371 feet, is exceeded by the barometrical refult 2 1 feet, 
though the circumftances were fuch as, in other cafes, 
generally make it fall fhort. 
At the bottom of the tables of computations I have 
occafionally fubftituted Mr. de luc’s equation for the 
air, in calculating one or more of the greateft heights, 
that the difference between the two methods might be* 
come more obvious. Thus the firfl: obfervation on Schi- 
hallien is defective feet; the mean of thofe on Tinto 
29 feet; Moel Eilio 41 feet; and Snowdon 81 feet. 
With refpedt to the refults that the rule produces on 
Mr. de luc’s heights it will be obferved, that it anfwers 
very well in the cold obfervations, which, with his rule, 
were often defective 60 or 70 feet; but gives too much 
in thofe that are hot. If, however, the whole of thefe 
hot obfervations had been included, the apparent error 
would have been lefs; for the mean defedf was taken at 
■~s for the hottefl temperature, whereas it fometimes 
amounts to T y^. On the height of the Dole the rule errs 
indefedt; and on the mean of Mr. de la caille’s ob- 
fervations, at the Cape of Good Hope, it exceeds the 
truth. By fubftituting Mr. de luc’s equation for the air, 
