448 Professor Forbes's Researches on Heat. 



a micrometer does the limb of an astronomical circle.* The re- 

 sult of this application of optical power is, that equally accurate 

 conclusions may be drawn within small limits of deviation, as if 

 the deviations were greater, and observed in the ordinary way. 

 This is important on several accounts, 1. The instrument is not 

 liable to those derangements which follow from exposure to con- 

 siderable heat, — derangements difficult to allow for, and which I 

 have not succeeded in obviating. 2. The value of the degrees is 

 more nearly uniform, and less liable to abrupt change ; so that (as 

 will presently be seen) within the narrow limits under which I am 

 accustomed to operate, the deviations are almost as the forces. 

 3. The motions of the needle being much more speedy and cer- 

 tain within small ranges (particularly in its return to zero) much 

 time is saved, and the consecutive observations are more accurately 

 comparative. 4. By the use of the telescope all parallax in read- 

 ing is avoided, and if a diagonal eye-piece be employed, the posture 

 is much less fatiguing than in any other method of observation. 

 6. Besides using this optical contrivance, I have increased the 

 delicacy of the instrument, by adding a conical reflector (seen at 

 M, Fig. 1), so as to concentrate nearly parallel rays upon the sur- 

 face of the pile. This contrivance is not my own. I saw one 

 in an instrument made on the model of M. Nobili's, in the 

 possession of M. Quetelet, at Brussels, in 1832, which was the 

 first multiplier I had seen. This reflector increases the sensibi- 

 lity of the pile to heat from a given source seven or eight times, 

 or in a proportion not very different from the area of its aperture 



* References io Fig. 1. — P, the thermal pile. KI, LH, the wires conveying 

 the electricity from the pile to the galvanometer. G, the galvanometer card, over 

 which the needle traverses. EF, the cover of the galvanometer, which has a plane 

 glass top. BC, a telescope, with a diagonal eye-piece, having an additional object- 

 glass at D, in order to give distinct vision of the galvanometer-needle and the scale 

 (see First Series, art. 5). A, the telescope-bearer, moving round a point concentric 

 with G M, the trumpet-mouthed reflector, applied to the pile (art. 6, below). 



