rtain criterion of the sensation of
heat with which human bodies are affected; for, as the presence and
perpetual succession of fresh air is necessary to our respiration, so
there is a species of tainted or stagnated air often produced by the
continuance of great heats, which, being less proper for respiration,
never fails to excite in us an idea of sultriness and suffocating
warmth, much beyond what the heat of the air alone would occasion,
supposing it pure and agitated. Hence it follows, that the mere
inspection of the thermometer will never determine the heat which the
human body feels from this cause; and hence also, the heat, in most
places between the tropics, must be much more troublesome and uneasy,
than the same degree of absolute heat in a high latitude. For the
equability and duration of the tropical heat contribute to impregnate
the air with a multitude of steams and vapours from the soil and
water; and many of these being of an impure and noxious kind, and
being not easily removed, by reason of the regularity of the winds
in those parts, which only shift the exhalations from place to place,
without dispersing them, the atmosphere is by this means rendered
less capable of supporting the animal functions, and mankind are
consequently affected by what they call a most intense and stifling
heat. Whereas, in the higher latitudes, these vapours are probably
raised in smaller quantities, and are frequently dispersed by the
irregularity and violence of the winds; so that the air, being in
general more pure and less stagnant, the same degree of absolute heat
is not attended by that uneasy and suffocating sensation.

This may suffice, in general, with respect to the present speculation;
but I cannot help wishing, as it is a subject in which mankind are
very much interested, especially travellers of all sorts, that it were
more thoroughly and accurately examined, and that all ships bound
to the warmer climates were furnished with thermometers of a known
fabric, and would observe them daily, and register their observations.
For, considering the turn to philosophical enquiries which has
obtained in Europe since the beginning of the eighteenth century, it
is incredible how very rarely any thing of this kind has been
attended to. For my own part, I do not remember to have ever seen any
observations of the heat and cold, either in the East or West Indies,
which were made by marines or officers of vessels, excepting those