HEAT. 



(Journal of Science, No. XVI.) which he 

 considers more convenient. One of the 



fid- 7. 



bulbs is made to stand higher than the 

 other, and the included liquid is alcohol 

 or ether coloured, which is made to boil 

 for the purpose of excluding the air, 

 previous to the closing of the instru- 

 ment by the blow-pipe. (Fig. 8.) 



Several instruments 

 have been made, to indi- 

 cate changes of tempera- 

 ture, upon the principle 

 of the unequal expansion 

 of different metals. Mr. 

 Crichton of Glasgow, 

 has combined small ob- 

 long plates of steel and 

 zinc : the compound bar 

 thus produced, is firmly 

 secured at one end to a 

 board ; the other end is 

 applied to a moveable 

 index, so that the whole 

 of the bending occasion- 

 ed by the superior ex- 

 pansibility of the zinc 

 over that of the steel, is 

 exercised in moving the 

 arms of the index along 

 a graduated arc, and 

 leaves them at the great- 

 est deviation to the right 

 or left of any observed 

 temperature. 



" An exquisite instrument," says Dr. 

 Ure, " on the same principle has been 

 invented by M. Breguet, member of the 

 Academy of Sciences, and Board of 

 Longitude of France. It consists of a 

 narrow metallic slip, about T 5 of an 

 inch thick, composed of silver and pla- 

 tina soldered together ; and is coiled in 

 a cylindrical form. The top of this spi- 



ral tube is suspended by a cross arm, 

 and the bottom carries, 'in a horizontal 

 position, a very delicate golden needle, 

 which traverses as an index on a gra- 

 duated circular plate. A steel stud 

 rises in the centre of the tube, to pre- 

 vent its oscillations from the central 

 position. If the silver be on the out- 

 side of the spiral, then the influence of 

 increased temperature will increase the 

 curvature, and move the appended 

 needle in the direction of the coil ; while 

 the action of cold will relax the coil, and 

 move the needle in the opposite di- 

 rection." The principle of these last- 

 mentioned contrivances is clearly the 

 same as that of Arnold's compensation 

 balance, already alluded to. 



Various modifications of the thermo- 

 meter have been introduced, for the 

 purpose of adapting it to particular pur- 

 poses, w r hich cannot here be described. 



Of the contrivances for measuring 

 high degrees of temperature, that of 

 Wedgewood has been the most in use : 

 its indications depend upon the contrac- 

 tion of pure clay when much heated. 

 This reduction of bulk is first observed 

 when the clay acquires a red heat, and 

 continues to increase until vitrification 

 ensues ; the contraction of volume 

 being permanent, and amounts, in the 

 whole, to about one fourth. In 

 order to take advantage of this pro- 

 perty of clay, Mr. Wedgewood con- 

 structed a guage of brass, consisting of 

 two straight pieces, two feet long, fixed 

 upon a plate, a little nearer to each 

 other at one end than at the other ; the 

 space between them at the widest end 

 being five-tenths of an inch, and at the 

 narrowest three-tenths. The converg- 

 ing pieces were divided into inches and 

 tenths of inches. The pieces of clay, 

 the contractions of which were to be 

 measured, were of a cylindrical form, 

 flattened on one side, and of such a size 

 as to be exactly adapted to the wider end 

 of the guage, so that it might slide far- 

 ther in, in proportion, to (he degree of 

 heat applied to it. 



The indications of this" instrument, 

 which he called the Pyrometer, from two 

 Greek words signifying measure of fire, 

 gave the comparative degrees of heat 

 produced in different processes ; but to 

 obtain the utmost information which 

 the instrument was capable of afford- 

 ing, it seemed absolutely necessary to 

 apply a scale to it, the degrees on which 

 should bear some certain proportion to 

 the degrees on the scale of Fahrenheit, 

 C 2 



