The various sizes of the bulbs of thermometers is another great cause of inequality, but which is easily reducible to a known law ; for since all the particles of the substance cxteris paribus require equal times to acquire a new temperature, the sen sibility will be inversely as the masses; but as such a change depends upon an equable effect produced by the extent of surface exposed to the acting me dium, it will be directly as the surfaces. But as in spherical bulbs the surfaces vary as the squares of the diameters, but the mass as the cubes, the sen 2 sibility will be as the power of the diameter, that is, inversely as the diameter simply.
But external causes, or those independent of the expanding substance itself are more complex and diversified. They may perhaps be classified in the following way.
1. Radiation, a solar, b Terrestrial.
II. Reflection of heat by buildings and other ob jects.
III. Absorption. a, By the wall or object to which the thermometer is attached. b, By the scale and mounting. c, By the colours of neighbouring of)! jects.
IV. Humidity. a, At the time of dew falling. b, By the proximity of water.
I. Of the existence and great force of radiation, both of heat from the sun impressing the thermo meter, which interrupts the free passage of its rays, and elevates the temperature above that of the at mosphere, and of heat from the earth in cold nights, which by transmitting rays to distant points of a clear sky, when the equilibrium is disturbed by the absence of the sun—it is not our present object to speak—but both are sources of error in observa tions conducted for other purposes than to measure them. The direct action of the sun's rays upon the naked bulb of the thermometer is much smaller than many persons imagine, for so vast a portion is reflected by the brilliant polish of the mercurial bulb, that it requires to be blackened to indicate the true effect, by absorbing the luminous and ca lorific rays. We have often seen a thermometer very little influenced by the direct rays of the sun, if his altitude was not great, and there was a breeze of wind. From direct radiation it is unnecessary to warn the attention of the observer, it is by reflec tion and absorption that its influence is so difficult to be avoided. It is not merely in ascertaining great colds, as at night,* that the influence of ter restrial radiation is sensible. Even where more than half the sky is sheltered from the instrument, and in ordinary summer weather, its influence must be avoided. This will be illustrated by Table II. in which a large thermometer hung at a window, in the usual way, is compared with one having the scale detached and laid among cptton on the win dow sill exposed to the northern sky. The differ ences are very striking, and show how much errors are to be avoided in experiments, even at ordinary windows. 'We shall quote some other observations
connected with terrestrial radiation, when we touch on the subject of Dew.
II. REFLECTION.—In making observations in towns this is one of the greatest difficulties to be struggled with, since opposite houses, and the street below, never fail to reflect great quantities of extraneous heat, even in the best exposures. It is very frequently to be observed, that from the posi tion of a thermometer in country houses,all the heat of the afternoon's sun is reflected from some pro jecting wall, yet the indications of the instrument are unsuspectingly observed to the last moment that the sun's rays do not touch the bulb. This is a fertile source of error to the amount of very many degrees, among ordinary observers—but in its more refined forms, reflection is an almost invincible ob stacle, even with the greatest care—the sun's rays arc reflected even from grass at great distances.t We have often seen thermometers hung at a north window, on the third floor from the ground, mate rially affected, in the middle of the clay, by the son's rays upon the grass, thirty feet below, and at a very considerable distance from the foot of the wall. The preventions of reflection are sufficiently simple in description, though very difficult to be obtained in practice. Double and independent coats of polished silver for the bulbs of thermome ters have, we believe, been occasionally employed to resist radiation in all its forms; but where accu rate experiments, on high temperatures, are to be made, we can recommend nothing equal to hang ing the thermometer under the branches of a thick tree, abundantly surrounded by others; at a win dow little can be done when the air is not in a tol erable state of motion; pasteboard screens against particular reflecting objects, such as flat roofs, we have sometimes used, with advantage. Register thermometers are particularly liable to the effects of reflection, and all other disturbing causes in the indication of maximum temperatures, which has often caused us great inconvenience; polished cop per cylinders, with free ventilation above and be low, in which the instrument is placed, have been recommended for this purpose, particularly in tropical climates, where from the verticality of the sun, perfect shade is peculiarly difficult to he ob tained. Reflection is so rarely separated from the influence of absorbed heat by surrounding objects, that the separation of them requires experiments peculiarly conducted, which, from the unparalleled unpropitiousness of the season, we regret not to have had an opportunity of completing. United with the effects of absorption, we shall presently give several examples. We have pleasure, how ever, in referring to the experiments of Captain Sabine and others, in Daniell's :Meteorological Es says on this subject.