[ 260 ] 
are bird’s Fahrenheits ; but, to the fixed thermo- 
meter, I have had a particular fcale applied, in 
which the fundamental interval, between melting ice 
and bird’s boiling point, is divided into 81 equal 
parts. The equation for the temperature of the quick- 
filver is one fathom for ever)/ degree, of this fcale, in 
the difference of temperatures. The thermometers, 
for the temperature of the air, have each a fcale, in 
which the fundamental interval between melting 
ice and bird’s boiling point, is divided into 120 
equal parts. The point of o is placed at the 5th 
of thele divifions above melting ice. If n be the 
mean height of the thermometer, in degrees of 
this fcale, at the two flations, the equation for the 
temperature of the air is — L. -|- or — , according 
as n is pofitive or negative. 
The place of the point of o upon the former 
fcale is indifferent. It was put very low, that the 
temperatures of the quicklilver, at both flations, 
might always be above it. The computation, how- 
ever, is rendered eafier by the tables now given, 
than it can be by any fuch peculiar divifions of the 
fcale. 
It is to be particularly obferved, that M. de luc 
always expofed the thermometer, by which he mea- 
fured the temperature of the air, to the fun, if it hap- 
pened to fhine; but then the ball of his thermometer 
was always quite detached from the frame, which is a 
neceffary precaution in this manner of ufing it. If 
the temperature of the air were meafured by a ther- 
mometer, expofed in the fhade, I cannot but think 
the quantity of the equation would be different. 
His 
