A contrivance, intermediate between Six's and Rutherford's has been more lately proposed by Dr. Traill. It is the minimum thermometer of Ruther ford, with a short piece of mercury A (Fig. 3.) in troduced above the index a, and the spirit again covering the mercury contains another index b. It is manifest that the mercury, by pushing back and forward these indices, marks the two extremes; be ing of steel, gilded by means of galvanism, they are brought back by the magnet. To this elegant in strument, we fear from the trials we have made, that there are likely to be strong objections; we have not merely found the mercury extremely li able to separate, but there is reason to fear, from the certainty that a film of spirit lines the whole interior of the tube, that by the efforts of its cor puscular attraction, and the difficulty of moving a sluggish column of mercury, however short, the position of the mercury, and consequently the true values of the degrees, will be changed.
Mr. Keith of Ravelstone§ has described a ther mometer which, by a mechanical contrivance, re gisters for any given time, in a continuous form, the flux of atmospheric temperature. His instrument has the form represented in Fig. 4. the bulb A down to the point is filled with spirit, where mer cury commences, which is continued up to the point Jr in the opposite leg. It bears a float, which by means of a marker b, indicates the temperature upon a scale affixed, and by a mechanical contri vance, leaves indices at the extreme points. By the adaptation of a pencil to the marker, and of a rota tory cylinder moved by clock work, in place of the scale, it is obvious that the atmospheric changes will be transferred to it for every instant of time. This mechanical invention is very elegant, but has never met with, and is not calculated for, general adoption. Such mechanically drawn charts might please the eye or the fancy of the general observer, but we think we may lay it down as a maxim in meteorological science, that little is to be done in the present state of its advancement, by any mere mechanical contrivances not of the simplest possi ble nature, or which extend beyond the limits of a glass tube hermetically sealed.
Le Chevalierli has proposed lately a plan of re gistration sufficiently simple in its idea, and which had long since occurred to us. It is the adaptation of an index to the metallic thermometers which have been described in another part of this article. The difficulty, however, is to find a method suffi ciently delicate not to impede the motion of these most susceptible instruments. No plan has been given for this.
We have already mentioned that, besides the re gistration of the greatest heat and cold, thermome ters have been proposed to record the temperature at any given epoch. Mr. Keith's, as we have seen, answers this purpose, but one more specifically for this end has been proposed by Mr. H. H. Black
adder. ¶ He has a thermometer A. Fig. 5. con structed exactly similarly to Cavendish's maxi mum," only that it contains mercury alone, and breaking the termination sharply over, fixes it into the case b without bringing it to a capillary termi nation, which we think a manifest defect. When held with the bulb A lowest, the column of mer cury may obviously lie made continuous; but if by a piece of clock-work the position he made hori zontal, as shown in the figure, the column will be interrupted, and form a mark of the height of the mercury at that moment, when compared with the attached common thermometer B. But if the tem perature were to rise after the given moment, the mercury would obviously be expelled from the end of the tube, to avoid which two hair pencils are contrived, at the moment of immersion, to drop an evaporable fluid on the bulbs, until they are ob served, when by adding the degrees vacant in tube Ab, to the height of the thermometer B, we have the temperature required for the given epoch. By terminating the tube of the thermometer A, in the manner shown in Fig. 6, it is obvious that the same effects will be obtained by a mere rotatory motion of the instrument, coinciding with the axis of the tube. We have not room to state our various objec tions to the instrument, but we conceive that the complexity of its construction, combining a double thermometer, a time-piece, and an evaporating ap paratus, is such as to prevent its adoption beyond a very confined sphere.
So much for self-registering thermometers ; we must now say a word or two on a modification of the common thermometer, in its most original form, which has been termed the differentia/ thermome ter. It is far from our intention to occupy our co lumns with any dispute regarding the true inventor of the instrument, which is exceedingly elementary in its principles, and is chiefly valuable from its applications. We will, however, say that we have examined the rare work of Professor Sturmius of Altdorff, entitled " Collegiurn Esperimentale Curl °sum," Nuremberg 1676, where the details and figures are so copious as to leave no doubt on the mind of the candid inquirer that Sturmius was the inventor of this modification of the thermometer. Professor Leslie and Count Rumford both revived it in 1804, under the titles of Differential Thermo meter and Thermoscope, two forms of which are shown in Plate DXXV. Figs. 7 and 8. Its principle consists merely in showing the difference of tempe rature between two media, in which the balls A and B are placed, and these being filled with air, the excess of expansion of air in the one above the other is marked by the motion of the coloured fluid which fills the greater part of the bent stem, the height of which, h, is referred to a scale attached.