410 An Essay on the Begree of Warmth of coloured Hays. 



equal to the complement of the latitude of the place ; and the 

 arm that moves the speculum acting in the direction of this 

 plane, will evidently cause it to follow the sun in its diurnal mo- 

 tion, whatever may be its declination. As the esperiments con- 

 cerning light and colours must be made in the dark, the whole 

 machine, when made use of for this purpose, must be shut up in 

 a case or box ; and we may have the rays thrown by means of 

 it in any required direction during the wliole of the time he is 

 above the horizon, if the situation be open ; or, if otherwise, as 

 long as the rays are not intercepted by surrounding objects. 



Another instrument of this sort is described by Mr. Benjamin 

 Martin, in page 46,9 of vol. ii. of his Mathematical Institutions, 

 which he considers as an improvement of one described by 

 -'I. Klingenstiern in the Petersburg Commentaries for 1747 and 

 1 748 : it may also be found in his Philosophia Britannica, p. 8f), 

 vol. iii. edit, of 1771: but the expense of such a machine it is 

 feared will prevent its being commonly applied, although it would 

 be extremely useful for moving telescopes round upon a polar 

 axis, in making observations of the heavenly bodies, and pur- 

 suing them, in an uniform manner, notwithstanding the earth's 

 diurnal motion. 



The late ingenious Mr. Ramsden, whose care in the construc- 

 tion, and accuracy in the execution of astronomical instruments, 

 could only be equalled by his genius and originality in inventing 

 them, had once an idea of adapting an instrument of this sort to 

 a clock that kept time very exactly, with a view to avoid the 

 necessity of using time in particular cases of astronomical ad- 

 measmements. 

 Christ's Hospital, June 7, 1815. 



LXXI. All Essay on the Degree of Whrmth of colovred Rays. 

 By M. I' Abbe Rochon, late Member of the Academy of 

 Sciences of Paris, &c. &c.* 



J.N this memoir we confine ourselves to examine, whether the 

 rays of light of different refrangibility produce degrees of heat 

 that differ in their degree of sensibility on the thermometer. 



This research requires without doubt that the colours should 

 be distinct and well separated ; but this analysis of them can- 

 not be made without diminishing prodigiously the intensity of 

 the light ; which, when thus decomposed, no longer gives any 

 sensible sign of warmth in our common thermometers. 



It is therefore necessary to resolve or to collect on the ball of 



* For some remarks on tliis pnper, see the preceding communication 

 finm Dr. Evans, page 401. 



a ther- 



