ain a Voyage across the Atlantic. 117 
had been made, that the method, which I then had fallen upon, 
was sufficiently exact. Some time elapsed in trials of it, be- 
fore such confidence was put in it, as led to register the obser- 
vations which were made. 
This very simple method of ‘measuring humidity of the 
atmosphere, consists in wetting. the bulb of the thermometer 
with water, and exposing it wet to the free access of air. The 
évaporation of moisture from the wet surface of the instrument 
reduces the temperature of it to the point at which deposition 
of dew takes place ; provided no extraneous substance be con- 
tiguous, from which heat might be derived to the evaporating’ 
moisture. On this account the thermometer employed must 
not have an attached scale in contact with the bulb. It is best 
graduated upon the glass of the tube. 
Reduction of temperature by evaporation of moisture is 
obviously a measure of the hygrometric state of the atmo- 
sphere : for, if the air be saturated with humidity, it produces 
no evaporation: if, on the contrary, it be dry, the cooling 
effect of the consequent evaporation is sensible; and the drier 
the air is, the greater .the coolness which is induced: the de- 
gree of refrigeration indicates the degree of dryness. 
Even upon a cursory view, coolness induced by evaporation, 
being noted with precision, is seen to be a hygrometric mea- 
sure. It not only is so, but it is an accurate one; since it 
exactly designates the point of deposition of dew: whence (the 
atmospheric temperature being at the same time known) the 
quantity and force of vapour present are inferable. For eva- 
poration of moisture from the surface of an isolated body con- 
tinues to depress its temperature, until it is cooled down to the 
verge of the point at which dew may be deposited. It is no 
further cooled, since condensation would else actually take 
effect, countervailing evaporation, and restoring at the instant 
the precise quantity of heat momentarily parted with. The 
cooling process does not stop before it reaches that limit, unless 
indeed moisture be deficient, and a portion of dry surface be 
exposed; in which case, as in that of contact of an extraneous 
12 
