M. The first kind of ABSORPTION of heat we have to notice, is that by walls and other surround ing objects generally. Any extensive and fixed ob ject in the vicinity of the thermometer, obviously tends to impart to it a constant share of heat, and subtract from it independent of the actual temper ature of the air. The heat of an inhabited house is sufficient to influence considerably the indications of low temperatures made at the window. In the course of the diurnal rise of heat, the temperature of the mass of the building being lower, absorbs from the adjacent film of air, and the thermometer placed in it, a part of its heat. This cause, how ever, is generally not much felt, since the influence of reflection, or radiation, soon raises the tempera tu•e of the mass, which forming a receptacle for the accumulation of solar heat vaguely distributed through the atmosphere, rises, if its means of ac quisition be considerable, to a temperature exceed ing, by several degrees, the maximum in the shade, and as the diurnal temperature descends, the dis proportionate slowness of the cooling of so great a mass, aided by new accessions of radiated heat, gives to the afternoon indications of the thermome ter, a more than due height, a eircumszance often of great inconvenience to the observers, especially as, in those seasons where considerable tempera ture becomes an object of scientific curiosity to measure, the sun's sweep in Azimuth in considera ble latitudes, is so great as to make a series of observations at one station, during a considerable part of the day, very difficult, and still greater un certainties are generally found in a new station; it will be found that the front of a building, towards the meridian, will, for several hours, after it has ceased to be affected with direct radiation, be un suited by absorption for delicate experiments.
The following tables will illustrate generally the above remarks, and some other effects both of re flection and absorption. Observations on solar radiation by a thermometer covered with black cloth, and exposed on bare soil, are generally ad ded, to indicate the state of the weather.
In Table III. No. 1, was a thermometer placed among thick trees in perfect shade. No. 2 was in the sun. No. 3 was a large thermometer hung at the east side of the airy and lofty north window already mentioned.
In Table IV. No. 1, is a thermometer placed among trees in perfect shade. No. 2, a thermome ter with metallic scale and glass case hung on a black painted pannel on the west side of a north window. No. 3, a large thermometer, mounted on metal, hung at the distance of one foot from the same window. No. 4, a small and delicate instru ment, at two feet distance. It is worth observa tion that the reflection from grass at 30 feet below, was so considerable, that No. 4 always indicated a temperature higher than No. 3, which was much more liable, in every respect, to the effects of ab sorption. The last observation at 6h. 15' is insert ed in confirmation of a remark we have already made upon the small influence of direct radiation upon a naked bulb when the sun is low.
The more general effect of approximation to a building, is an increase of temperature by the ab sorption and subsequent communication of heat by that body. Hence, unless the causes of direct re flection are preponderant, as in the last Tables, the thermometer nearest the wall indicates the most elevated temperature, as illustrated by the follow ing experiments.
curate temperature of the shade under trees, and No. 4, the indications of a thermometer covered with black cloth in the sun. In Table VI. some observations on humidity by a moistened thermom eter are added.
The three thermometers used were constructed for the purpose, of small size and flattened bores, with light paste-board scales attached. No. 1 hung freely at the window sill already mentioned. No. 2 at 1 ft. 3 in. from the wall, and No. 3 at ft. or double that distance. It will be observed that the height of No. 1 almost constantly exceed ed that of the others by a less or greater quantity. hut that the two last agreed perfectly till the shade of No. 3 was partially removed as the day advanc ed, a slight heat being communicated by the inter ruption of some of the rays by the frame to which the thermometers were attached; for the mere pas sage of the solar beams communicates no heat without the intervention of some object, and the direct distance of the grassy flat below, upon which they fell, was perhaps 40 feet. Had the day been unfavourable for the equilibrium of tem perature, the differences would have been more con siderable, but the wind being brisk from the N. W. diminished the sources of error. When the wind proceeds from points opposite to the sun at the hot part of the day, the instruments placed in the shade are of course most exposed to its influ ence, which often counterbalances all trifling causes; of course, in the N.E. point this reaches a maximum. The two following Tables will de monstrate its influence in annihilating the effect of absorption last illustrated, while they will show the constancy of the difference between observa tions made in contact with a building, and one de fended from radiation in all its forms by the shelter of thick trees, the instruments being within not much exceeding 100 yards from each other. No. 1 was the large thermometer already mentioned, hung one foot from the window. No. 2 a small and delicate instrument at 22 feet. No. 3 the ac The second kind of absorption we have mentioned is caused by the scales and mounting of thermome ters, which is often done in a very unskilful way, and in the measurement of high temperatures is as fertile a source of error as any other. Not merely do the scales of thermometers, when large, act as increasing the mass of matter to be heated or cooled, but, as we have already explained in the case of buildings, magnify the influence of radiation and reflection, by accumulating the calorific rays which find their way to the instrument; and if the scale be metal, as is frequently the case, its rapid conduct ing power facilitates the communication. This is our principal objection to metallic scales. The adoption of glass cases also, to protect the instru ment from the effect of weather, is, to the last de gree, prejudicial in retarding an equilibrium of temperature; the small hole at the bottom of such cases being a very tardy medium of communica tion. These remarks will he confirmed by the fol lowing table of the indications of three instruments, extremely differently mounted, and placed together at a free exposure at a window. The total effect of absorption will be seen by a comparison with a thermometer in absolute shade, No. 4.