1909.] SEE— THE PAST HISTORY OF THE EARTH. 127 



The vapors which arise from the sun, the fixed stars, and the tails of the 

 comets may meet at last with, and fall into, the atmosphere of the planets 

 by their gravity. 



Cheseaux was the first to express the view that the heavenly- 

 spaces are not perfectly transparent, but that light suffers a certain 

 amount of absorption or extinction in passing over great distances. 

 (Cf. L. de Cheseaux, " Traite de la Comete qui a paru en 1743 et 

 1744," 8°, Lusanne & Geneva, 1744, p. 223.) This account of 

 Cheseaux was written five years before the promulgation of Euler's 

 views, and it is uncertain to what extent, if at all, Newton and 

 Cheseaux had influenced Euler in reaching the conclusion that the 

 planets suffer resistance in their motion about the sun. 



The extracts from Euler's letters are as follows : 



I. First Letter: 



XXH. Monsieur le Monnier writes to me that there is, at Leyden, an 

 Arabick manuscript of Ibn Jounis (if I am not mistaken in the name, for it 

 is not distinctly written in the letter), which contains a history of Astro- 

 nomical observations. M. le Monnier says, that he insisted strongly on 

 publishing a good translation of that book. And as such a work would 

 contribute much to the improvement of Astronomy, I should be glad to see 

 it published. I am very impatient to see such a work which contains obser- 

 vations, that are not so old as those recorded by Ptolemy. For having 

 carefully examined the modern observations of the sun with those of some 

 centuries past, although I have not gone further back than the 15th cen- 

 tury, in which I have found Walther's observations made at Nuremberg; 

 yet I have observed that the motion of the Sun (or of the Earth) is sensibly 

 accelerated since that time; so that the years are shorter at present than 

 formerly ; the reason of which is very natural, for if the earth, in its motion, 

 suffers some little resistance (which cannot be doubted, since the space 

 through which the planets move, is necessarily full of some subtile matter, 

 were it no other than that of light), the effect of this resistance will grad- 

 ually bring the planets nearer and nearer the sun; and as their orbits thereby 

 become less, their periodical times will also be diminished. Thus in time 

 the earth ought to come within the region of Venus, and in fine into that 

 of Mercury, where it would necessarily be burnt. Hence it is manifest 

 that the system of the planets cannot last forever in its (present) state. 

 It also incontestibly follows that this system must have had a beginning; 

 for whoever denies it must grant me, that there was a time, when the earth 

 was at the distance of Saturn and even farther, and consequently that no 

 living creature could subsist there. Nay there must have been a time when 

 the planets were nearer to some fixt stars than to the Sun ; and in this case 

 they could never come into the solar system. This then is a proof, purely 

 physical, that the world in its present state, must have had a beginning, and 



PROG. AMER. PHIL. SOC. XLVIII. I9I I, PRINTED JULY 8, I9O9. 



