Mr. Smcaton remarks, that the micrometer is best judged of by the hearing, rather than by the sight or feel ing ; and that by this method, he found it practicable to repeat the same measurement several times without dif fering from itself above the 20000th part of an inch.
The following are the results obtained by Mr. Smeaton from an increase of heat corresponding to 180° of Fahren heit, or the difference between freezing and boiling water.
These results agree very well with those made by Mr Ellicott. See the Phil. Trans. 1754, vol. xlviii.
6. Description of Ferguson's Lever and 117teel Pyro meters.
The lever pyrometer, invented by Mr. Ferguson, is so simple, as scarcely to require a figure for its illustration. Upon a fiat piece of mahogany are fixed brass studs, on which the metallic bar is placed. One end of a bar bears against a lever of the second kind, at a point very near its fulcrum. The other end of this lever, w hich is bent, bears against another lever, and very near its fulcrum ; and the other end of this last lever is the index, which has a gra duated arch under it. The small expansion of the metallic bar is magnified by the first lever in the proportion of the distances of the point of pressure from its plane, and from its other extremity ; and this magnified effect is again mag nified by the other lever, so that an expansion of the 400th part of an inch corresponds to a whole inch on the scale. This pyrometer is liable to the objection that the distance of the points of pressure from the fulcrum and extremity of each lever is variable during the experiment. Sec Fer guson's Lectures, v. i. p. 14, Ed. 1824.
Mr. Ferguson's wheel pyrometer differs from the lever one in the substitution of wheels and pinions in place of levers. The metallic bar bears against the end of a short bar which advances between rollers. This short bar has fifteen teeth on one side,- which act upon the leaves of a pinion of twelve teeth, on the axis of which is fixed a wheel of one hundred teeth, which teeth again take into the leaves of another pinion of ten leaves, on the axis of which is placed another wheel of one hundred teeth, which again take into the leaves of a third pinion of ten leaves, whose axis carries the index. It is evident from a slight calcula
tion, that one degree of the circular scale divided into 360 parts, corresponds to the 45,000th part of an inch. By means of a piece of watch: spring connected with the second pinion by a silk thread, the wheels are pulled back again when the bar contracts, and the teeth of the wheels are kept in contact with the leaves of the pinions. See Ferguson's Lectures, vol. i. p. 301. Both these pyrome ters are more fitted for the exhibition of the principle and effects of a pyrometer in a public lecture, than for taking any nice measures of expansion.
7. Description of Ramsden's Microscopic Pyrometer.
The pyrometer of Mr. Ramsdell, which we propose to describe, derives its name from two microscopes attached to it, by which the expansions are measured. The ap paratus consists of a strong deal frame, five feet long, nearly twenty-eight inches broad, and about forty-two inches high. The bar' of metal, the expansion of which is to be measured, and which may be even two feet long, is placed in a copper trough or boiler more than five feet in length, and filled with water. Beneath the trough are twelve spirit lamps, whose flames heat the water in the trough to the boiling point, and consequently the me tallic bar is raised to the same temperature. Parallel to the copper trough, and at a little distance from it, are placed two other wooden troughs full of water, in each of which is placed a cast iron prismatic bar. At the ex tremities of the bars, and perpt ndicular to them, are fixed the two microscopes above mentioned. One of these mit roscopes has only a simple mark or point in the field of view, but the other is furnished with a wire microme ter, similar to the wire micrometer described under Mr