As it is strictly a thermometer of differences, it refers merely to the relative elasticities of the air in the two balls, and being hermetically sealed, is free from the common defects of the air thermo meter.* The most important application of the dif ferential thermometer to practical meteorology is the hygrometer invented by Mr. Leslie, on the Hut tonian principle, of the difference between the tem perature of a dry and moistened surface, a ease exactly fitted for the interposition of the differen tial plan. We are happy to refer to a very elegant self-registering hygrometer on this principle, in vented by the Reverend Mr. Gordon of Kinfauns, and described in the article METcOxoLoGY, Vol.
XIII. p. 167, in which the principle of Rutherford'sXiii. p. 167, in which the principle of Rutherford's minimum thermometer is elegantly applied to both the maximum and minimum indications of dryness.
The differential thermometer has been made the basis of several other instruments by Professor Les lie, adapted to meteorology, hut which have not come into general use, and which it would be be yond the object of this article to detail.
A new application of the simple thermometer has recently been proposed by Baron Fourier,t which he names a Thermometer of Contact. It is shown in Fig. 9, and consists of a bulb and tube AB filled with mercury, as usual. By means of a case CDEF, it is surrounded with mercury retained by a leather bag, a b, and being placed with the bottom resting on any substance a, whose conduct ing power is to be examined, the time of the de scent of the temperature from a certain point to which it has been artificially raised, to that of the substance under experiment, expresses, by an in verse ratio, its conducting power, which it is the object of the instrument to determine.
The thermometric properties of bodies have been applied to various mechanical contrivances of great ingenuity. One or two, which may rather be con sidered new forms of the instrument, than any thing else, deserve a place here. Dr. Cummings of Chester proposed a statical thermometer, which, by being constructed on a large scale, might open and shut windows of rooms or hothouses as the temperature became elevated or depressed above or below any standard temperature. A similar idea occurred, we believe, many years ago, to one of our most original philosophers, Sir James Hall, Bart. who, in the course of his long series of admirable experiments, which required a steady and intense temperature for a considerable period, contrived a valve for his furnaces which by its own pyrometric action, was opened or closed as the temperature increased or declined. The instrument of Dr. Cummings consisted of a large glass or iron ma trass filled with air, but containing mercury in its stem, which was inverted into a cistern containing the same fluid. The matrass was supported from above by a cord over a pulley connected with the sash of a window, nicely balanced by it, so that it will easily be seen how, when the weight of the counterpoise was diminished by the expulsion of mercury from the stem as the excluded air increased in volume by temperature, the sash was let down from the top, and a communication with the exter nal air effected; the reverse of course taking place as the temperature declined.
The balance thermometer of Bewley is repre sented at Fig. 10. It consists of an elongated bulb of alcohol A, which fluid, in the expansion of the tube at B, is succeeded by mercury continuing along the tube ff, and has free motion in a recepta cle at c. The whole being suspended from a frame
b c, by the point a, to which knife edges are ad justed, it is manifest that the expansion of the al cohol will, by expelling part of the mercury from the bulb B into C, destroy the equilibrium of the machine, and the quantity of the deviation may be measured by the divisions on the brass scale e, moved by the pinion a, until the balance is restored. The machine, however, is most obviously applica ble as a motive agent for mechanical purposes, such as the opening and shutting windows. Long before we were acquainted with Mr. Kewley's instrument, it occurred to us that such a contrivance would be applicable to the purposes of self-registration, but we believe it has never been put in practice.
Such we consider to be a pretty full detail of all contrivances of merit for the improvement and mo dification of the thermometer. We have stated the principal disadvantages of each, as they occurred, generally from our own experience. Register ther mometers are the most imperfect, and open the widest field for ingenious adaptations, yet when we consider the very numerous experiments that have been made, and the few attempts at novelty, which of late years have been practically successful, we are tempted to consider much farther improvement in the theory as vain. There arc, however, very great desiderata in the workmanship of these in struments, and much also to be looked for from ob servers, and especially that the instruments should be more widely diffused. They are generally made too large, or, if smaller, act imperfectly. In com mon day and night thermometers the objection of size is comparatively unimportant, since the maxi mum and minimum points of temperature are usu ally sensibly stationary, till almost any instrument has had time to acquire it; yet, even for such obser vations, mum improvement is requisite, and how much more for those more refined inquiries which demand great sensibility in the indication of maxi ma and minima in short intervals; more especially in exploring the temperature of the atmosphere by balloons, or in hourly observations of the highest and lowest temperature. The clumsiness of most register thermometers is by no means an inherent defect, as many would have us believe ; by proper care and skill in the manufacture, they may be made of great fineness, even where indices are re quired. On the Cavendish plan of capillary termi nations with reservoirs, there is in some cases no limit to the degree of tenuity: we have very recently had a thermometer for maximum temperatures made with mercury alone, and having an attached mercurial thermometer precisely similar, to ascer tain the values of the height of the mercury, made on the most delicate scale, of which the thermome ter may, in some sense, be said to be capable, so that the bore containing mercury is little more than visible to the eye, the length of the degrees being considerable, and the whole calculated for the nicest experiments ; it appears to work extremely well. For general adoption, however, we are here dis posed to recommend Rutherford's thermometer, except that they are ill suited for carriage, a defect applying also to all thermometers containing mixed fluids.