meafuring Heights with the Barometer. 755 
vature may be fuppofed to change fail near the furface 
of the earth, and differ infeniibly from a flraight line at 
great heights above it. 
With regard to the latitudinal equation, the fame 
principle of heat and moifture feems to make it proba- 
ble, that fuch will become neceffary in operating with 
the barometer; for it is well known, that there is a great 
degree of humidity in the air between the tropics; and, 
on the contrary, that the polar atmofpheres are very dry. 
The heat and moifture being greateil at the equator, there 
the elafticity or equation will likewife be the greateil at 
the level of the fea; and the zero of the fcale will neceffa- 
rily defcend to a lower point of the thermometer, than 
that to which it correfponds in middle latitudes. As the 
elafticity of the air at the level of the fea, or equal 
heights above it, with the fame degree of heat, will al- 
ways be proportionable to the quantity of moifture dif- 
folved in it, therefore it will gradually diminifh from the 
equator towards the poles ; that is to fay, the zero of the 
fcale will afcend in the thermometer, coincide with the 
3 ad degree in the middle latitudes, and, in its motion 
upwards, will give the equation to be applied with the 
contrary flgn in high latitudes. Hence I infer, that every 
latitude, climate, or zone, will not only have its particu- 
lar zero, but alfo its particular curve, whofe ordinates 
will 
