312 On the Velocity of the calorific I\ays 



It shall next be attempted to explain the principles of ati 

 experiment applicable to the same purpose, and which does 

 not seem to be altogether beyond our reach. Though great 

 difficulties may stand in the way of an adequate apparatus, 

 yet at any rate it may not be amiss to expound the theory 

 of the method, so as to have it in reserve. The history 

 of science affords not a few instances of great difficulties 

 of a practical kind being at last surmounted. 



It is well known to astronomers how, by Dr. Bradley's 

 noble discovery of what has been called the aberration of 

 the fixed stars, the velocity of their light has been deter- 

 mined. The observations of the same astronomer prove 

 also the remarkable fact, that the velocity of all star-light 

 is the same. From this circumstance it might have been 

 concluded, upon the strongest grounds of analogy, that the 

 light of our sun is emitted with equal rapidity. But pre- 

 vious to the discovery of aberration, the velocity of the 

 sun's light, as reflected to us from Jupiter's satellites, was 

 determined by Romer in a different way; and the near 

 agreement between this and that of star-light makes it still 

 more reasonable to believe that the velocity of the sun's 

 emitted light makes no exception to the general law. 



It must therefore be so astonishingly rapid, as to carry the 

 rays over a space little short of two hundred thousand miles 

 in a single second of time. Still, however, rapid as this is, 

 the velocity of the earth in its orbit is found to bear some 

 sensible proportion to it. In consequence of this, it is de- 

 monstrable that we can never behold the sun in his true 

 place in the ecliptic, but always removed from it towards 

 the west, or contrary to the order of the signs, by an angle 

 of twenty seconds, corresponding to what has been called 

 the aberration of his light. 



Were the sun's light to be emitted with only orte half or 

 one third of its present velocity, the corresponding aberra- 

 tion or apparent change of place towards the west, as is 

 well known, would be doubled or tripled ; the centre of his 

 disk all the while still keeping in the ecliptic. 



In like manner, were the sun all at once to emit an addi- 

 tional set of rays equally refrangible, suppose of a deep 

 violet colour, but with one tenth part only of the velocity 

 belonging to his present white light, the centre of this vio- 

 let disk would appear to' us more westerly on the ecliptic 

 than that of his usual white disk, by an angle of three mi- 

 nutes ; that being the difference of aberration correspond- 

 ing; to the supposed difference of velocity of the two kinds 

 of light'. These two equal round disks, therefore^ each of 



2 ' whose 



