76 MR. T. HOPKINS ON IMPROVEMENT 
The principal instruments used in meteorological inves- 
tigations are the barometer, which may be considered to 
measure correctly the various alterations that occur in the 
amount of atmospheric pressure, the dry thermometer, 
and the wet bulb thermometer. In addition to these the 
dew-point, or point of condensation of vapour is ascer- 
tained, from which is deduced the tension, or pressure, of 
vapour. The barometer shows the total atmospheric pres- 
sure, but that pressure is known to be made up of two 
parts,—one the pressure of the gases, and the other that of 
the vapour which is diffused through them ; and in inves- 
tigating the subject it is necessary that we should know 
how the separate pressures of their constituents are 
ascertained, in order that we may judge of the degree 
of reliance that may be placed on the results that are 
obtained. 
It has been found by experiment that aqueous vapour 
loses its aeriform state, and is converted into water, when 
exposed to a sufficient degree of cold, or, in other words, 
when heat is sufficiently abstracted from it by reduction 
of temperature. And the more dense the vapour, the 
more readily does a reduction of temperature convert 
a part of the vapour into liquid. Hence it is seen 
that the varying quantities of aqueous vapour found in 
the atmosphere may be condensed by different degrees of 
cold; and the particular temperature at which the vapour 
is converted into water indicates the degree of abun- 
dance, or the density, in which it exists in any part of it. 
The temperature of liquefaction, or the dew-point, there- 
fore shows the quantity of vapour present in the atmos- 
phere. And each quantity has a particular weight and 
elastic force, with which it presses on the earth, and of 
course on the mercury of any barometer placed on its 
surface. This force is generally called the tension of 
vapour, and it indicates the quantity of vapour in the 
atmosphere, at any and every period of time. 
