THERMOMETER AND PYROMETER. 

 Fig. 32. 



escape into the empty part of th 

 From the position of both thermoi 

 it is obvious, that to bring both indices 

 to the surface of the respective fluids, it 

 is only necessary to incline the instru- 

 ment toward C ; and it is thus prepared 

 for a fresh observation. 



The accuracy of Rutherford's thermo- 

 meters depends on the ease with which 

 they are constructed, and the applica- 

 tion of a due correction for the inequa- 

 lities of expansion between them ; this 

 can be more readily accomplished 

 than with Six's thermometer ; because 

 the indications of each fluid are inde- 

 pendent of each other. The discre- 

 Eancies between both thermometers 

 ave been carefully examined by De 

 Luc,* and more lately by De Wildt, of 

 Hanover ;f the results will be given in 

 the sixth chapter, from which the cor- 

 rection can be applied. Such a correc- 

 tion will render them applicable to the 

 nicest meteorological observations of 

 maxima and minima. For more ordi- 

 nary purposes they are very convenient, 

 as not being easily deranged, and being 

 adjusted, for each observation, with the 

 utmost facility. 



5. In the Transactions of the same 

 society,^ we find another register ther- 

 mometer, by Mr. Alexander Keith, a 

 gentleman of great mechanical inven- 

 tion, and long an active member of that 

 society. It is represented in our Jig. 33 ; 

 where a b is a glass tube, 14 inches 

 long, and inch in calibre, sealed at the 

 top, and below communicating with a 

 bent tube b, d, 7 inches long, and 0.4 

 inches in diameter, open at the top, 

 where it is cemented to a metallic plate 

 e, which supports the ivory scale e, e, 

 6f inches long. From a to b, the tube 

 is filled with highly rectified alcohol, 

 and from b to c with mercury. At c is 

 a conical float of ivory or glass, resting 

 on the surface of the quicksilver, and 



Recherches sur les Mod. de 1'Atmosphfcre. 



f Jameson's Edin. Phil. Journal for October, 1826. 



^ Edin. Phil. Trans, vol. iv. 



' I 



supporting a kneed wire h, intended for 

 moving two indices of black silk i, k t 

 that slide along the fine gold wire g, f, 

 as will be readily seen from the figure. 



To prepare the instrument for obser- 

 vation, the indices are drawn, by means 

 of a crooked wire prepared for the pur- 

 pose, till they touch each side of the 

 knee of the float wire. It is obvious 

 that, as the heat alters the dimensions 

 of the column of spirit in a, b, the mer- 

 cury will rise or fall in the small tube, 

 and the float swimming on the surface 

 of the mercury will raise or depress the 

 knee h, which will move the indices ac- 

 cordingly on the wire g,f. The instru- 

 ment is defended from wind or rain by 

 the glass case I, I, which, by means of 

 its metal collar, fits tight on e, and is 

 only removed to adjust the indices. 



This instrument, it is true, is influ- 



