1910] on Lowell Observatory Photographs of tlie Planets. 819 



and the darker the gaps between seems inferable. But thej are not 

 as our clouds. With us the heat that causes cloud comes from with- 

 out ; with Jupiter from within. Sun-occasioned the one, self -evolved 

 the other. We have visual evidence of this internal heat of Jupiter 

 in the cherry red that tinges his darker belts as if we there looked 

 down into the seething cauldron below. We have theoretic proof of 

 it, too, in the oblateness the disk presents taken in connection with 

 what we know to be the planet's mean density. In two articles 

 shortly to appear in the Philosophical Magazine those who care for 

 mathematics will find that heat alone enables Jupiter to keep his 

 youthful figure, and furthermore that his shape shows him to consist 

 of a comparatively small kernel wrapped in a huge husk of cloud. 

 Even those who do not care for the oldest of the sciences must admit 

 a certain grandeur in it when theory can thus plumb depths experi- 

 ment may never fathom. 



These belts have another peculiarity. Their several parts are 

 travelling at idiosyncratic rates. With them it is a go-as-you-please 

 race, in which each outruns or falls behind its neighbour. On this 

 interesting subject we owe most to your fellow countryman, Mr. 

 Stanley Williams, who for some years has acted as timekeeper and 

 referee of this Jovian family contest. In future he will have no 

 mean rival in the photographic plate. Xot that it sees as well, but 

 that it may be measured at leisure by any investigator who likes. 



There is one feature you will mark in the photographs which 

 has had a long and eventful history. I refer to the great red spot. 

 Detected in 1879, it lasted as such to within a few years. Eather 

 a long life for a hole in the clouds ! Xow, properly speaking, we 

 see only the grave in which it lies buried, the oval shell it once 

 occupied. But these same photographs were in a sense the means of 

 bringing its cradle also to light. Sixty years ago, a cycle of Cathay, 

 Sir William Huggins made a fine series of drawings of the planet, 

 and on receiving the present pictures was struck by the resemblance 

 of the two. In consequence he sent me prints of his. On scanning 

 them, my eye was caught by an oval placed as the present one lies. 

 Clearly it was the cradle prepared already for the great red spot 

 twenty years in advance. He had been present before its birth, as 

 he is still, happily, present after its demise. 



The third point we may mention in these photographs is their 

 revelation of the equatorial wisps. Some years ago Mr. Scriven 

 Bolton detected a most curious set of markings lacing Jupiter's 

 bright equatorial belt. His discovery met with the usual approved 

 disapprobation which has been the orthodox reception of astronomical 

 advance since Galileo's time. Were a discovery to be hospitably 

 hailed it would prove disconcerting to the discoverer, who would 

 instantly suspect something wrong. Eventually the subject was 

 referred to us for corroboration. This we were able promptly to 

 secure. A singular phenomenon they proved to be, criss-cross fila- 



