743 Col. roy’s Experiments far 
gree on each; and the French toife being the 25th, will 
have its zero nearly at the 57th degree: about which 
temperature the expanfion of air appears, from the ex- 
periments, to be at its maximum. From that point, 
therefore, the equation will diminifh both ways, though 
by a quicker progreffipn for condenfation, than it doth 
for dilatation. 
Having thus compared, in a general way, the refults 
of the Britifh obfervations with thofe of Mr. de luc, 
pointed out what feem to be the chief caufes of the con- 
ftant defe£t found in his rule, and thereby obtained, it is 
hoped, fome corrections tending to improve the theory 
of the barometer, when applied to the meaiurement of 
heights in middle latitudes ; it remains to ftiew the prin- 
ciples, whereon the table for the equation of the air hath 
been conftruCted. Previoufly however to this, it may be 
proper to compare, with as much brevity as podible, 
thefe obfervations, with others that have been made To- 
wards the Pole and at the Equator: from which it will 
appear probable, that the rule which aniwers in middle 
latitudes, will not in the frigid and torrid zones. 
In i 773 5 Captain phipps, now Lord mulgrave, com- 
manding two of his Majefty’s fhips then lent on difcove- 
ries towards the North Pole, meafured geometrically, with 
g;reat care, the height of a mountain in Hakluyt’s Ifland 
near 
