VOL. LVir.] PHILOSOPHICAL TRANSACTIONS. 457 



motion in 15 days is 14° 17'. The spots traverse the like area in 1** O'' 50™ 

 nearly. Hence, if the earth stood still, the spots would be hid only la** 23'' 10™, 

 and their whole sidereal period being 25"* 5*^ they would be visible 11"* 5*^ 50"", 

 and the time of their occultation would exceed the time of their appearance by 

 a"* 17'' 20™. Hence the arc AC is less than the arc abc, by the motion of 

 2^ n^ 20™, that is, by 38° 52' 56*. And the semicircle being a mean 

 arithmetic between ac and abc, ac will be less than the semicircle by half as 

 much; that is by 1 9° 26' 28". Hence each of the angles gsa, hsc is 9° 43' 14*. 

 Tlie angle bsd = dTS = 16' l" 27'". Therefore dsA is 9" 27' 12". Hence SA 

 = 1.013767 such parts as sd is 1. 



The distance therefore of these spots from the centre of the sun, is 1.01 3767 

 semidiameters of the sun, and their distance from his surface is, in decimal 

 parts of his semidiameter, .013767. Hence it is evident that the height of 

 the solar spots above the surface of the sun, is above 54 such parts, as bear each 

 to the sun's semidiameter, the proportion of one Paris mile to the semidiameter 

 of the earth, which is that of 1 to 3923 nearly. The height of our atmosphere 

 is generally reckoned about 50 miles. That of the lowest clouds fall short of 

 one mile. The whole height of our atmosphere therefore is, at least, 50 times 

 that of our highest clouds. If the whole height of the sun's atmosphere bear as 

 large a proportion to the height of these solar spots or clouds (and I think the 

 proportion is likely to be much larger), the height of the sun's atmosphere is 

 not less in proportion to his semidiameter, than 54 times that of the earth's, 

 and exceeds two-thirds of his semidiameter, being in decimal parts thereof 

 -.68835. 



The probability seems to be, that the height of the sun's atmosphere is 

 almost double of this ; for I question whether the mean height of our clouds 

 exceeds \ a Paris mile. The solar spots therefore are 108 times as high in 

 proportion ; and then, supposing as before, that the whole height of the sun's 

 atmosphere bears the same proportion to the height of his spots, as the whole 

 height of our atmosphere to the mean height of our clouds, the sun's atmosphere 

 will be 108 times as high in proportion to his semidiameter, as ours is, and will 

 rise to the distance of more than -^ of his semidiameter from his surface. Let 

 philosophers consider, whether these indications of the vast height of the sun's 

 atmosphere give any degree of probability to a conjecture of Sir Isaac Newton's, 

 that the dissipation of the sun's substance, which might be expected to ensue 

 from his intense heat, may in great part be prevented by the prodigious 

 pressure of the incumbent atmosphere. 



The height of the atmosphere of Venus is considerably greater, according to 

 the observations of Mayer and Rohlius, than they imagined. Rohlius follows 

 Cassini in the estimation of the sun's apogee semidiameter, which Cassini over- 



VOL. XII. 3 N 



