Besides this estimation of the heat of a place, by taking the medium for a considerable time together, there is another circumstance which will still augment the apparent heat of { 172} the warmer climates, and diminish that of the colder, though I do not remember to have seen it remarked in any author. To explain myself more distinctly upon this head, I must observe that the measure of absolute heat marked by the thermometer is not the certain 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, supposing it pure and agitated, would occasion. 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 it follows, too, that 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 these being, many of them, 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 with what they stile a most intense and stifling heat: whereas in the higher latitudes these vapours are probably raised in smaller quantities, and the irregularity and violence of the winds frequently disperse them, so that, the air being in general pure and less stagnant, the same degree of absolute heat is not attended with 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, especially travellers of all sorts, are very much interested, that it were more thoroughly and accurately examined, and that all ships bound to the warmer climates would furnish themselves with thermometers of a known fabric, and would observe them daily, and register their observations; for considering the turn to philosophical inquiries which has obtained in Europe for the last fourscore years, it is incredible how very rarely anything of this kind { 173} hath been attended to. As to my own part, I do not recollect that I have ever seen any observations of the heat and cold, either in the East or West Indies, which were made by mariners or officers of vessels, except those made by Mr. Anson's order on board the _Centurion_, and by Captain Legg on board the _Severn_, which was another ship of our squadron.