Ice melts... Wedgwood thinks the freezing point of vapour a little higher. [Young's Nat. Phil. SECTION II. Fariation of Local Heat.-BY JAMES SIX, Esq. To investigate the variation of local heat, I made the following experiments. On Sept. 4, 1783, I placed thermometers in 3 dif ferent stations; one on the top of the high tower of Canterbury cathedral, about 220 feet from the ground; another at the bottom of the same tower, at about 110 feet; and a 3d in my own garden,* not more than 6 feet from the ground. They were all carefully exposed to the open air in a shady northern aspect; the lowest was as little liable to be affected by the reflection of the sun's rays as the elevation would permit, the second still less, and the highest not at all. They thus remained in their several places, where I visited them daily for 3 weeks, and minuted down the greatest de gree of heat and cold that happened each day and night in their respective stations, by a peculiar thermometer. By these observations it appears that, notwithstanding some irregularities, the heat of the days at the lowest station always exceeded that at the middle, and still more the heat at the upper Situated not far from the cathedral, at the extremity of the buildings on the north side of the city.-Orig. station. As in many instances the higher regions of the atmo sphere have been found to be colder than the lower, and the thermometer in the garden was more liable to be heated by the reflection of the sun's rays from the earth than the upper ones, a dif ference of this kind might have been expected. But I was greatly surprized to find the cold of the night at the lowest, not only equal to, but very frequently exceeding the cold at the higher stations. As I wished to know, whether these variations would continue the same in winter, when the weather was colder; and whether a ther mometer, placed at some distance from the city, having an elevation equal to that on the top of the cathedral tower, would agree with it; on Dec. 19, 1783, I disposed the 3 thermometers in the following manner: one in my garden; one on the top of the high tower, as before; and the third on the top of St.Thomas's hill, about a mile distant from the city, where, at 15 feet from the ground, it was nearly level with that on the cathedral tower. The weather at this time proving cold, favoured the experiment; and I now found the several thermometers nearly agreeing with each other in the day-time; but in the night, the cold at the lower station exceeded the cold at the higher ones rather more than it did in the month of September, when the weather was warmer. At the time of taking these thermometrical observations, I like. wise noted the different dispositions of the atmosphere in other respects such as the pressure, moisture, and dryness of the air; force and direction of the winds; quantity of rain; whether the appearances of the sky were clear or cloudy, &c. as I apprehended the local variation of the thermometers might, in a certain degree, correspond with some particular change in the state of the atmo. sphere. The event answered my expectation in a singular manner in respect to the nocturnal variation; for it generally happened, that when the sky was dark and cloudy, whatever was the condition of the atmosphere with regard to the other particulars above enumerated, the thermometers agreed pretty nearly with each other; but, on the contrary, whenever the sky became clear, the cold of the night at the lowest station in the garden constantly exceeded the cold at the top of the cathedral tower, where the instru ment was placed 220 feet from the ground, entirely exposed to the open air, wind, dews, and rain, in a shady northern aspect. The local variations in the day time seemed to be regulated by the general degree of heat only, without being affected by any other particular disposition of the atmosphere, or the clearness or cloudiness of the sky, as the nocturnal variations were. In the month of September, when the glasses rose from 60° to 70°, the heat at the lower station constantly exceeded the heat at the upper station; and in some measure proportionably, as the weather was hotter.* In December and January, when from below 30° they seldom rose to 40°, the local variation in the day-time nearly ceased, or was found in very small degrees inclining sometimes one way, some. times the other. That the clearness of the sky should contribute to the coolness of the air in the night, is not at all surprizing; but that, whenever the sky becomes clear, the cold should seem to rise from the earth, and be found in the greatest degree, as long as it continues clear, in the lowest situation, seems a little extraordinary: this however appeared to be the case, both in the warmer and in the colder weather, during the whole time these observations were taken. About noon, on the 3d of January, the sky becoming clear, the air got cooler; and going into my garden, about 8 in the evening, I perceived the surface of the ground, which had been wet by the rain in the forenoon, began to be frozen. Looking immediately at the thermometer, I saw the mercury at 3340; and observing a piece of wet linen hanging near the glass, not 5 feet from the ground, I took it into my hand, and found it not in the least frozen; by which it appeared, that the degree of cold which had frozen the surface of the ground, had not then ascended to the glass, nor to the linen, and consequently had not been communi. cated to the air 5 or 6 feet above the earth. The next day I found, as expected, a considerable local variation; the index for the cold of the night in the garden being at 32°, that on the hill being at 354, and that on the top of the tower at 37°. Probably the weather did not continue clear the whole night; if it had, it is likely the degrees of cold would have been found proportionably greater at every station. On the morning of the 4th there fell a As the heat at the lower station exceeded the heat at the upper ones, when the weather was hot; and equally so, whenever the sky was cloudy, as well as when it was clear; it appears, that the glass at the lower station was not materially affected by the reflection of the sun's rays from the earth, as at first I ap prehended it would be,-Orig. misty rain, which continued only till noon, when the sky became clear again, and continued so till the 7th; during which time the nocturnal heights of the thermometers differed considerably from each other; but on the sky's becoming cloudy, the local variation ceased. By experiments of this kind it may possibly in some measure be found, how far evaporations from the earth, at certain times, or vapours ascending, descending, or meeting, in different parts of the atmosphere, may increase or diminish the heat of the air in those places or whether different degrees of heat and cold, sub. ject however to change, may not be found in different strata of air, or vapour, floating in different parts of the atmosphere; or in what degree and proportion the cold increases at different altitudes, and in different seasons of the year; whether the cold, which is known to be very intense in the summer time on the tops of high mountains, receives a proportional increase, or be not less subject to variety by the return of winter and summer, night and day, than what we experience in the plains below. [Phil. Trans. 1784. Mr. Six afterwards continued a similar series of experiments throughout an entire year, in order to ascertain how far the results might coincide with those of the preceding paper, which were confined to a part of the autumn and of the winter. To this end, as he tells us, in another paper communicated to the Royal Society in the year 1788, he suspended proper thermometers in a shady northern aspect, in the open air, at different heights; one in the garden at 9 feet, and another in the Cathedral Tower 220 feet from the ground; continuing his journal, with the omission of a few days only, from July 1784 till July 1785. The result entirely corresponded with what was before observed respecting the nocturnal diminution of heat, and the particular state of the atmosphere requisite to produce it. From the 25th to the 28th of October, the heat below in the night exceeded, in a small degree, the heat above, at which time there was frequent rain, sometimes mingled with hail. From the 11th to the 14th, and also on the 31st, there was no variation at all; during which time likewise the weather was rainy; all the rest of the month proving clear, the air was found colder below than it was above, sometimes 9 or 10 degrees. On cloudy nights, in June, the lowest thermometer |
