an instrument for measuring the expansion of bodies by heat. See Chemistry, no 103. Mutschensbroeck, who was the original inventor of this machine, has given a table of the expansion of the different metals in the same degree of heat. Having prepared cylindric rods of iron, steel, copper, brass, tin, and Pyrometer and lead, he exposed them first to a pyrometer with one flame in the middle; then with two flames; and successively to one with three, four, and five flames. But previous to this trial, he took care to cool them equally, by exposing them some time upon the same stone, when it began to freeze, and Fahrenheit's thermometer was at 32 degrees. The effects of which experiment are digested in the following table, where the degrees of expansion are marked in parts equal to the \( \frac{1}{550} \) part of an inch.

| Expansion of | Iron | Steel | Copper | Brass | Tin | Lead | |--------------|------|-------|--------|-------|-----|------| | By one flame | 80 | 85 | 89 | 110 | 153 | 155 | | By two flames placed close together. | 117 | 123 | 115 | 220 | 274 | | | By two flames 2½ inches distant. | 109 | 94 | 92 | 141 | 219 | 263 | | By three flames placed close together. | 142 | 168 | 193 | 275 | | | | By four flames placed close together. | 211 | 270 | 270 | 361 | | | | By five flames. | 230 | 310 | 310 | 377 | | |

It is to be observed of tin, that it will easily melt when heated by two flames placed together. Lead commonly melts with three flames placed together, especially if they burn long.

From these experiments, it appears at first view that iron is the least rarefied of any of these metals, whether it be heated by one or more flames; and therefore is most proper for making machines or instruments which we would have free from any alterations by heat or cold, as the rods of pendulums for clocks, &c. So likewise the measures of yards or feet should be made of iron, that their length may be as nearly as possible the same summer and winter.

The expansion of lead and tin, by only one flame, is nearly the same; that is, almost double of the expansion of iron. It is likewise observable, that the flames placed together, cause a greater rarefaction than when they have a sensible interval between them; iron in the former case, being expanded 117 degrees, and only 109 in the latter; the reason of which difference is obvious.

By comparing the expansions of the same metal produced by one, two, three, or more flames, it appears that two flames do not cause double the expansion of one, nor three flames three times that expansion, but always less; and these expansions differ so much the more from the ratio of the number of flames as there are more flames acting at the same time.

It is also observable, that metals are not expanded equally at the time of their melting, but some more some less. Thus tin began to run when rarefied 219 degrees; whereas brass was expanded 377 degrees, and yet was far from melting.

Mr Ellicot found, upon a medium, that the expansion of bars of different metals, as nearly of the same dimensions as possible, by the same degree of heat, were as follow:

- Gold, Silver, Brass, Copper, Iron, Steel, Lead, 73 103 95 89 60 56 149

The great difference between the expansions of iron and brass has been applied with good success to remedy the irregularities in pendulums arising from heat. See Pendulum.

Mr Graham used to measure the minute alterations, in length, of metal bars, by advancing the point of a micrometer-screw, till it sensibly stopped against the end of the bar to be measured. This screw, being small and very lightly hung, was capable of agreement within the three or four-thousandth part of an inch. On this general principle Mr Smeaton contrived his pyrometer, in which the measures are determined by the contact of a piece of metal with the point of a micrometer-screw.

The following table shows how much a foot in length of each metal grows longer by an increase of heat, corresponding to 180° of Fahrenheit's thermometer, or to the difference between freezing and boiling water, expressed in such parts of which the unit is equal to the 10,000th part of an inch.

1. White-glass barometer tube, - 100 2. Martial regulus of antimony, - 130 3. Blistered steel, - 138 4. Hard steel, - 147 5. Iron, - 151 6. Bismuth, - 167 7. Copper hammered, - 204 8. Copper eight parts, with tin one, - 218 9. Calt brass, - 225 10. Brass sixteen parts, with tin one, - 229 11. Brass-wire, - 232 12. Speculum metal, - 232 13. Spelter folder, viz. brass two parts, zinc one, - 247 14. Fine pewter, - 274 15. Grain tin, - 298 16. Soft folder, viz. lead two, tin one, - 301 17. Zinc eight parts, with tin one, a little hammered, - 323 18. Lead, - 344 19. Zinc or spelter, - 353 20. Zinc hammered half an inch per foot, - 373

We shall close this article with a brief description of a pyrometer lately invented by M. De Luc, in consequence of a hint suggested to him by Mr Ramden. The basis of this instrument is a rectangular piece of deal-board two feet and a half long, 15 inches broad, and one inch and a half thick; and to this all the other parts are fixed. This is mounted in the manner of a table, with four deal legs, each a foot long and an inch and a half square, well fitted near its four angles, and kept together at the other ends by four firm cross-pieces. This small table is suspended by a hook to a stand; the board being in a vertical situation in the direction of its grain, and bearing its legs forward in such a manner as that the cross-pieces which join them may form a frame, placed vertically facing the observer. This frame contains a microscope, which is firmly fixed in another frame that moves in the former by means of grooves, but with a very considerable degree of tightness; Pyrometer,ness; the friction of which may be increased by the pressure of four screws. The inner sliding frame, which is likewise of deal, keeps the tube of the microscope in a horizontal position, and in great part without the frame, infomuch that the end which carries the lens is but little within the space between the frame and the board. This microscope is constructed in such a manner as that the object observed may be an inch distant from the lens; and it has a wire which is situated in the focus of the glasses, in which the objects appear reversed. At the top of the apparatus there is a piece of deal, an inch and a half thick and two inches broad, laid in a horizontal direction from the board to the top of the frame. To this piece the rods of the different subdivisions, whose expansion by heat is to be measured, are suspended: one end of it slides into a socket, which is cut in the thickness of the board; and the other end, which rests upon the frame, meets there with a screw, which makes the piece move backward and forward, to bring the objects to the focus of the microscope. There is a cork very strongly driven through a hole bored vertically through this piece; and in another vertical hole made through the cork, the rods are fixed at the top; so that they hang only, and their dilatation is not counteracted by any pressure. In order to heat the rods, a cylindrical bottle of thin glass, about 21 inches high, and four inches in diameter, is placed in the inside of the machine, upon a stand independent of the rest of the apparatus. In this bottle the rods are suspended at a little less than an inch distance from one of the insides, in order to have them near the microscope. Into this bottle is poured water of different degrees of heat, which must be stirred about, by moving upwards and downwards, at one of the sides of the bottle, a little piece of wood, fastened horizontally at the end of a stick: in this water is hung a thermometer, the ball of which reaches to the middle of the height of the rods. During these operations the water rises to the cork, which thus determines the length of the heated part; the bottle is covered, to prevent the water from cooling too rapidly at the surface; and a thin cage of brass prevents the vapour from fixing upon the piece of deal to which the rods are fixed.