meafuring Heights with the Barometer. 725 . 
feafons of 1774 and 1775 afforded a few on the frnall 
heights in and near the metropolis; but the bell I have, 
been furnifhed with are thofe which Dr. lind, a flitted 
by Mr. hoy, was fo obliging as to make on Arthur’s Seat 
near Edinburgh ; and thofe which Captain calderwood. 
has fi nee favoured me with on the Cairn-hills, being a. 
part of the Pentland range to the South-wett of that city. 
By comparing thefe fets of obfervations together, it 
appeared from all of them, that when the air was at or 
near the freezing temperature, the logarithmic differ- 
ences gave the real height, in Englifh fathoms and thou- 
fandth parts, without any equation; and when confidera- 
bly below that point, the equation was to be fubtradfed, or 
applied with the fign - inftead of + . It was farther per- 
ceived, that the fame general conclufion might be drawn 
from the coldeft, not only of the Sun-rifing, but even 
of the ordinary obfervations on Saleve, fome reduction of 
the temperature being in certain cafes made, on account 
of the expofure of the thermometer to the Sun’s rays : 
hence I was led to fuppofe, that the morning obferva- 
tions, inftead of being made exceptions from the rule, , 
were thofe, which, it might be prefumed, would form 
the belt bafis for deducing the equation depending on 
the heat of the air; becaufe the mean temperature of the 
column was then found to differ leaft from that of its 
ext re- 
