the calorific Powers of the different prismatic Rays. 409 



the differently coloured rays of light after having been decom- 

 posed by means of the prism . To this claim lie certainly appears 

 entitled, notwithstanding the great want of agreement which at 

 present apparently exists between his results and those of others. 



We have many instances on record, of things that appeared at 

 first sight to be of a very discordant nature, proving after further 

 investigation to be perfectly reconcileable with themselves and 

 with truth; an instance of which may be taken from a subject 

 stated further back, of the discovery of the progressive motion of 

 light by Romer. The several discussions that took place re- 

 specting it from the year 1675 to 1707, by Messrs. Cassini, 

 Maraldi, and the discoverer, show how difficult it is to bring the . 

 scattered fragments of a noble edifice to agree, and build them 

 up again upon a firm and immoveable foundation. When all 

 the parts are found and made out that belong to the theory of 

 light, heat, and colours, then perhaps, and not till then, these of 

 the Abbe Rochou may find their place among the rest in the 

 general arrangement of the whole. 



The ratio of the warmth of clear red to the most lively violet, 

 of 8 to 1, which he deduces from his experiments, does not ap- 

 pear to be admissible, unless the point of the scale of his 

 thermometer showed the commencement of all accession of ca- 

 loric in the fluid of which it v.as composed. Thus the ratio of 

 5-117 to 40T21 may be expressed in round numbers by that of 

 1 to 8; but if the point of cojigelation, or whatever other point 

 may be deemed that where the first accession of heat begins, be 

 S degrees below th« zero of the thermometer, then the ratio of 

 8-f-5-117 to 2 + 40-121, is no longer that of i to 8, but ap- 

 proaches so much the nearer to a ratio of equality as o increases. 

 Let a and |3 be the two original lieights ; then the excess of the 



latter ratio above the former will be ■ }'„, when zero is reclcon- 



ed 8 degrees below. 



The heliostata, which he regrets the want of, is an instrimient 

 contrived by Gravesande, and very fully described by him in his 

 Mathematical Elements of Natural Philosophy, vol. ii. page 107, 

 edit, of 17-^ 7- The intention of it is to avoid the doulile incon- 

 venience arising from the obliquity of the rays, and tlic sun's 

 diurnal motion. On account of the obliquity of the rays, some 

 experiments can only be made at certain hours ; and others can- 

 not l)e made at all in particular places, although exposed to the 

 sun a considerable portion of the dav. This machine consists of 

 two principal parts, a plane metallic speculum, and a clock, by 

 whicli it is kept directed always towards the sun. A metallic 

 speculum is used to avoid the double reflection from a glass one. 

 The plane of the clock is inclined to the horizon in an angle 



e(iual 



