1773.] TO HUMPHR V M A R S II A I. L. 5} g 



them vast clouds of smoke and soot, arising from (lie consuming 

 fuel on the surface, which clouds at length take fire again on their 

 edges, consuming and daily diminishing till they totally disap- 

 pear. Others think them spots of the surface, in which the fire 

 has been extinguished, and which by degrees arc rekindled. It 

 however, remarkable, that tho' large spots are seen gradually to 

 become small ones, no one has observed a small spot gradually to 

 become a large one ; at least, I do not remember to have met with 

 such an observation. If this be so, it would seem that tiny are 

 suddenly formed of the full size. And perhaps if there were more 

 such constant and diligent observers as you, some might happen to 

 be observing at the instant such a spot was formed, when the ap- 

 pearances might give some ground of conjecture by what means 

 they were formed. 



The professor of astronomy at Glasgow, Dr. WlLSON, has a new 

 hypothesis. It is this: that the sun is a globe of solid matter, all 

 combustible perhaps, but whose surface only is actually on lire to 

 a certain depth, and all below that depth unkindled; like a log of 

 wood whose surface, to half an inch deep, may be a burning coal. 

 while all within remains wood. Then he supposes, that by souk 

 explosion, similar to our earthquakes, the burning part may be 

 blown away from a particular district, leaving bare the unkindled 

 part below, which then appears a spot, and only lessens as the 

 fluid burning matter by degrees flows in upon it on all sides, and 

 at length covers or rekindles it. He founds this opinion on certain 

 appearances of the edges of the spots, as they turn under the sun's 

 disk, or emerge again on the other side ; for, if they are such 

 hollows in the sun's face, as he supposes, and the bright border 

 round the edges be the fluid burning matter flowing down the banks 

 into the hollow, it will follow, that while a spot is in the middle of 

 the sun's disk, the eye, looking directly upon the whole, may dis- 

 cern that border all round; but when the hollow is moved round to 

 near the edge of the disk, then, though the eye, which now news 

 it aslant, can see full the farthest bank, yet that which is near. 

 is hidden and not to be distinguished. And when the same spol 

 comes to emerge again on the other side of the sun, the bank which 

 before was visible is now concealed, and that concealed which before 

 was visible, gradually changing, however, till the spot reaches the 

 middle of the disk, when the bank all round may be seen as before. 

 Perhaps your telescope may be scarce strong enough to observe 

 this. If it is, I wish to know whether you find the same appear- 



