24 



THERMOMETER AND PYROMETER. 



cut in the board a b to the front of the 

 frame ; this is movable by means of 

 the screw A, and is perforated by a 

 piece of cork m firmly driven into it 

 until level with its lower surface. The 

 cork is then pierced vertically to receive 

 the glass rod o o o, which is thus sus- 

 pended in a thin cylindrical glass jar 

 p, 21 inches high, and 4 inches in 

 diameter, filled with water. The glass 

 rod is the standard of comparison, and 

 to it is attached a rod s s, of the metal 

 to be tried, by two connected rings r r, 

 which are tightened on the rods by two 

 screws. Another set of rings v is ap- 

 plied higher up; but through this the 

 rod s s freely slides, while it firmly clips 

 the glass rod by means of a screw. A 

 delicate thermometer hangs in Ihe centre 

 of the jar p to note the temperature of 

 the water, which is occasionally agitated 

 to secure uniformity of temperature by 

 the rod q q. A syphon z to draw off 

 the water completes the apparatus.* 



In using this pyrgmeter, warm water 

 is poured into the jar, in order to heat 

 the rods ; the rods are adjusted to the 

 focus of the microscope by the screw 

 n; the thermometer gives the degree 

 of heat employed ; and, by means of 

 marks on the bars, their relative ex- 

 pansion is given in divisions of the 

 micrometer, the value of which is 

 known by previous experiments. The 

 connection of the rods is more dis- 

 tinctly seen at A ; but it is unnecessary 

 to give a more minute description of an 

 instrument which has been superseded 

 by the more accurate and more elegant 

 contrivance of Ramsden, so elaborately 

 detailed by General Roy, to which we 

 shall presently advert. 



11. From experiments with this instru- 

 ment, De Luc ingeniously applied a cor- 

 rection to the scale of barometers for 

 temperature, by what, in the same 

 paper, he calls " metallic thermome- 

 ters" The scale of the barometer 

 was fixed on a bar of metal of known 

 expansibility, so as to raise the scale in 

 exact proportion to the expansion of 

 the mercury ; and thus the mere in- 

 spection of the barometric scale will 

 give the true height, without the trouble 

 of applying the equation or formula of 

 correction for temperature, as in ordi- 

 nary observations. 



12. We come now to certainly the most 

 complex, but the most perfect of all 

 contrivances for determining the rela- 



* Phi). '1 rans. vol. Ixviii. part i. p. 43?. 



tive expansions of solids, the micro- 

 scopic pyrometer of Ramsden, contrived 

 by that eminent artist, for determining 

 with the utmost possible precision, the 

 expansibility of the rods employed by 

 General Roy, in the geodesical opera- 

 tions that are the foundation of the 

 great trigonometrical survey of Britain. 

 Fig. 18 contains plans and sections of 

 this beautiful contrivance, and although 

 we do not propose to enter into a mi- 

 nute detail of the different parts of the 

 instrument, a general description will 

 show, to those who have not consi- 

 dered such subjects, the nice precau- 

 tions which are necessary to accuracy 

 in like operations, while it explains its 

 construction. Ramsden's pyrometer is 

 attached to a strong and well joined 

 deal table, or frame 5 feet long, 28 

 inches broad, and 42 inches high ; 

 of which an end elevation is seen fig. 

 18, B ; the plan of its top will be best 

 understood from an inspection of A. 

 a b and c d are troughs of deal, 

 (firmly screwed to the table) 3 inches 

 in diameter, and a little longer than the 

 frame ; a b projects a little over the 

 table, but c d is in a line with the 

 frame, as may be seen at B. Each 

 trough contains a cast iron prism, 1| 

 inch on each side, firmly fixed in the 

 troughs, at the ends a and c, by means 

 of brass collars embracing the prisms, 

 and tightened by screws as at G, while 

 the ends b and d pass freely through 

 loose collars, without any shake, when 

 their dimensions are altered by tempe- 

 rature. The prism a b is called the 

 eye prism ; because it carries at each 

 end the eye-pieces of the microscopes 

 Imn, and o p r; which are figured on 

 a larger scale at F and E. The other 

 prism c d is called the mark prism ; 

 because it carries at one end the mark I, 

 and at the other cross wires H; efis a cop- 

 per boiler 2| inches wide, and 3| deep, 

 rather shorter than the wooden troughs. 

 The centre of the boiler, or rather of 

 the object lens standing perpendicular 

 to it, is 5.81 inches from the cross 

 wires of the mark in c d, and 20.33 

 inches from the wire of the micrometer 

 attached to the corresponding eye-piece. 

 The boiler rests on five small rollers, 

 seen in the enlarged section D. The 

 boiler, like the troughs, has acock to the 

 right hand ; and in the plan A, it is 

 represented with a bar in it, to show 

 the position of the rods to be tried. 

 The water in the boiler is heated by the 

 12 spirit lamps gggg, standing on four 



