PYROMETER. instrument was in vented by Mussehenbroek for Measuring the effects produced in the dimensions of solid bodies by the Application of heat; but the signification of the teem has since been ex tended so As to include those instruments the object of which is to measure all gtadations of temperature itheVe those which can be indi cated by the meiturial thermoMeter. We Will briefly describe five or Silt varieties.
insschenbrock's Pgrontetei• Consisted of a metallic bat, About six inchet hi length, one extremity of which was fixed, while the 'other was left flee to advance as the metal elongated froth the effect of several spirit laMps placed beneath, which, at enah o rinient, woe charged with the same qiutlitity of highly rec tified spirit Of wine. The advance of the moveable extremity gave motion to a pinion and wheel, the latter of which drote an index over a dradortted circle, each degree of which cbrt'esponded to a linear expansion of some deterniimite fraction of an inch. Deseguliers afterwards substituted fine cords and friction rollers for the wheel and pinion.
Borda's Pyrontcte•.—The rods employed by Borda in measuring the base-line of the great French Survey consisted of a rule of brass laid upon a somewhat longer rule of platinum and attached at one extremity. The portion of the platinum rule not covered by the one of brass was divided into millionths of its entire length, and further subdivided by means of a vernier and microscope adjusted to the extre mity of the brass rule. The value of each of these divisions having been previously ascer tained, by first surrounding the compound rule with melting ice, and then immersing it in boiling water, it was only necessary to ob serve the indications of the vernier in order to apply the requisite correction for reducing the length of the rod to the standard tempe rature.
Banaden's Pyrometer.—For low tempera tures, the contrivance of Ramsden was em ployed by General Roy in determining the expansion of the rods used in measuring the base on Hounslow Heath for the Trigonome trical Survey. The rod was immersed in a'
trough of water, and over each extremity was placed a microscope, to which a slow motion could be given in the direction of .the length of the rod by means of a fine micrometer screw. The temperature of the _water was then gradually raised, till a thermometer placed in the trough indicated an advance of 10°, 20°, 30°, or any required number of de grees. The consequent elongation of the rod destroyed the coincidence of its extremities with the lines of collimation of the micro scopes, which was re-established by turning the micrometer screws, and carefully noting the number of turns and fractions of a turn necessary for that purpose ; when, the value in parts of an inch in each turn being pre viously known, a direct measure of the expan sion was obtained, free from the errors of a system of levers or of a train of wheels and pinions.
Wcdgwood's Pyromeler.—The property of alumina whereby it undergoes a diminution of bulk when heated, was employed by Wedg wood as a measure of high temperatures. His pyrometer consisted of cylinders. of fine white clay, and an apparatus for accurately measur ing their length. This apparatus consisted of a metallic plate, upon which were fixed two brass rules slightly inclined to each other. The rules used by Wedgwood were 24 inches long, and divided into 210 equal parts. The distance between the rules at one extremity was three-tenths and at the other five-tenths of an inch ; consequently the difference be tween their distances at any two consecutive divisions was the 1200th part of an inch. The clay cylinders were first baked at a red heat (about Fehr.), and then reduced to ex actly five-tenths of an inch in length, so as to fit the first division of the scale. When after wards exposed to a greater heat they under went contraction, and the amount of this con traction was determined by observing the division of the scale corresponding to their diminished length.