Hitherto I have considered the temperature of the air all the year through, and the gross estimations of heat and cold which every one makes from his own sensations.  But if this matter be examined by means of thermometers, which are doubtless the most unerring evidences in respect to the absolute degrees of heat and cold, the result will be indeed most wonderful; since it will appear that the heat in very high latitudes, as at Petersburgh for instance, is, at particular times, much greater than any that has been hitherto observed between the tropics.  Even at London in the year 1746, there was a part of one day considerably hotter than was at any time felt in one of the ships of our squadron in the whole voyage out and home, though four times passing under the equator; for, in the summer of that year, the thermometer in London, graduated according to the scale of Fahrenheit, stood at 78 deg., and the greatest observed heat, by a thermometer of the same kind in the same ship, was 76 deg., which was at St Catharines in the latter end of December, when the sun was within about 3 deg. of the vertex.  At St Petersburgh, I find by the acts of the Academy, in the year 1734, on the 20th and 25th of July, that the thermometer rose to 98 deg. in the shade, or 22 deg. higher than it was found to be at St Catharines; which extraordinary degree of heat, were it not authenticated by the regularity and circumspection with which the observations appear to have been conducted, would appear altogether incredible.

If it should be asked, how it comes then to pass, that the heat, in many places between the tropics, is esteemed so violent and insufferable, when it appears, by these instances, that it is sometimes rivalled, and even exceeded, in very high latitudes, not far from the polar circle?  I shall answer, That the estimation of heat, in any particular place, ought not to be founded upon that particular degree of it which may now and then obtain there; but is rather to be deduced from the medium observed during a whole season, or perhaps in a whole year; and in this light, it will easily appear how much more intense the same degree of heat may prove, by being long continued without remarkable variation.  For instance, in comparing together St Catharines and St Petersburg, we shall suppose the summer heat at St Catharines to be 76 deg., and the winter heat to be only 56 deg..  I do not make this last supposition upon sufficient authority, but am apt to suspect the allowance is full large.  Upon this supposition, therefore, the medium heat all the year round will be 66 deg.; and this perhaps by night as well as by day, with no great variation.  Now, those who have attended to thermometrical observation will readily allow, that a continuance of this degree of heat for a length of time, would be found violent and suffocating by the generality of mankind.  But at Petersburg, though the heat, as measured by the thermometer, may happen to be a few times in the year considerably higher than at St Catharines, yet, at other times, the cold is intensely sharper, and the medium for a year, or even for one season only, would be far short of 60 deg..  For I find, that the variation of the thermometer at Petersburgh, is at least five times greater, from its highest to its lowest point, than I have supposed it to be at St Catherines.[2]