PENDING PROBLEMS OF ASTRONOMY. 59 



The problem of the sun's peculiar rotation and equatorial accelera- 

 tion appears also a most important one, and is still unsolved. Proba- 

 bly its solution depends in some way upon a correct understanding of 

 the exchanges of matter going on between the interior and the surface 

 of the fluid, cooling globe. It is a significant fact (already alluded to) 

 that a similar relation appears to hold upon the disk of Jupiter ; the 

 bright spots near the equator of the planet comisleting their rotation 

 about five minutes more quickly than the great red spot which was 

 forty degrees from the equator. It is hardly necessary to say that an 

 astronomer, watching our terrestrial clouds from some external station 

 (on the Moon, for instance), would observe just the reverse. On the 

 Earth, equatorial clouds complete their revolution tnore slowly than 

 those in our own latitude. Our storms travel toward the east, while 

 the volcanic dust from Krakatoa moved swiftly west. We may at 

 least conjecture that the difference between different planets some- 

 how turns upon the question whether the body whose atmospheric 

 currents we observe is receiving more heat from without than it is 

 throwing off itself. Whatever may be the true explanation of this 

 peculiarity in the motion of sun-spots, it will, when reached, probably 

 carry with it the solution of many other mysteries, and will arbitrate 

 conclusively between rival hypotheses. 



The periodicity of the sun-spots suggests a number of important 

 and interesting problems ; relating, on the one hand, to its mysterious 

 cause, and, on the other, to the possible effects of this periodicity 

 upon the earth and its inhabitants. I am no " sun-spottist " myself, 

 and am very doubtful whether the terrestrial influence of sun-spots 

 amounts to anything worth speaking of, except in the direction of 

 magnetism. But all must concede, I think, that this is by no means 

 yet demonstrated (it is not easy to prove a negative) ; and there cer- 

 tainly are facts and presumptions enough tending the other way to 

 warrant more extended investigation of the subject. The investiga- 

 tion is embarrassed by the circumstance, pointed out by Dr. Gould, 

 that the effects of sun-spot periodicity, if they exist at all (as he main- 

 tains they do), are likely to be quite different in different portions of 

 the earth. The influence of changes in the amount of the solar radia- 

 tion will, he says, be first and chiefly felt in alterations and deflections 

 of the prevailing winds, thus varying the distribution of heat and rain 

 upon the surface of the earth, without necessarily much changing its 

 absolute amount. In some regions it may, therefore, be warmer and 

 drier during a sun-spot maximum, while in adjoining countries it is the 

 reverse. 



There can be no question that it is now one of the most important 

 and pressing problems of observational astronomy to devise apparatus 

 and methods delicate enough to enable the student to follow promptly 

 and accurately the presumable changes in the daily, even the hourly, 

 amounts of the solar radiation. It might, perhaps, be possible even 



