54>6 Prof. Forbes*s Researches on Heat. Second Series, 



of a solid press, and clamping a little arm carrying the 

 telescope, and centred at a point in the prolongation of a 

 vertical line, passing through the centre of the card of the 

 galvanometer, to a shelf above. The little arm bearing the 

 telescope, therefore, traverses the divided part of the galvano- 

 meter card, just as a micrometer does the limb of an astrono- 

 mical circle. The result 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 derange- 

 ments which follow from exposure to considerable heat, — de- 

 rangements difficult to allow for, and which I have not suc- 

 ceeded 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 certain 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 tele- 

 scope all parallax in reading is avoided, and if a diagonal eye- 

 piece be employed, the posture is much less fatiguing than in 

 any other method of observation. 



Besides using this optical contrivance, I have increased the 

 delicacy of the instrument, by adding a conical reflector so as 

 to concentrate nearly parallel rays upon the surface of the 

 pile. This contrivance is not my own. I saw one in an in- 

 strument made on the model of M. Nobili's, in the possession 

 of M. Quetelet, at Brussels, in 1832, which was the first mul- 

 tiplier I had seen. This reflector increases the sensibility 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 

 to that of the pile"*. The length of the conic frustum which 

 I employ is If inches, and its aperture If inches. 



It is well known that the deviations of galvanometers are 



• Suppose that we wish to have a conical reflector A BCD, such that the 

 whole of the parallel rays 

 which fall upon it shall 

 reach some part of the sur- 

 face AB of the pile, which 



is all that we want, we have .^^---''' 



this simple construction. ,^---"''' 



Let the length of the trum- F ^''^^ 



pet-mouth A E be given. 

 Make F B equal and pa- 

 rallel to it. Join F A, and ^ 

 prolong the line to D, then is DAE the greatest inclination that the sides 

 of the cone can have to answer the purpose intended. 



