90 



SCIENCE. 



[N. S. Vol. XVII. No. 420 



there is no law connecting rotation period 

 with the latitude, except we find this period 

 of 9^^ 50™ more commonly between the 

 limits of ■ — 8 and + 11 degrees, whereas 

 the longer period is distributed indiscrim- 

 inately over the surface of the whole planet 

 as far as 38 degrees latitude. 



The question has sometimes been raised 

 as to whether the phenomena on Jupiter 

 were periodic. The inclination of Jupiter's 

 equator to its orbit being only three de- 

 grees, any periodicity due to the revolu- 

 tion of Jupiter around the sun should 

 recur at intervals of about twelve years, 

 but from the motions which I have shown 

 for the displacement of the belts in lati- 

 tude there does not seem to be any regu- 

 larity in the period. I presume any 

 periodicity is of the same nature as we 

 have in the meteorology of the earth. 

 We have, of course, a sequence in the sea- 

 sons and a sequence in weather conditions, 

 but our sequence in weather conditions does 

 not follow any regularity, and if changes 

 on Jupiter are due to meteorological causes, 

 we should not expect to find any definite 

 period. 



The application of photography to astro- 

 nomical observations has been of great 

 value in various directions, but up to the 

 present time it has been of no benefit in the 

 study of planetary details. Photographs of 

 the planet Jupiter have been made since 

 1880 at different times, but they only show 

 the simple outline and some of the con- 

 spicuous markings. The scale of photo- 

 graph is so small that it cannot be used 

 with any degree of success for determining 

 position on the disk. There is no question, 

 however, that if we are ever able, by in- 

 creasing the sensitiveness of our plate, to 

 make an enlarged photograph of Jupiter 

 or Mars such as is seen through the tele- 

 scope with the eye, it would be a great ad- 

 vance, and it would enable us to decide very 



many questions, which are now impossible 

 owing to the limited time that we are able 

 to study the object under consideration, 

 due to the rapid motion of the planet on its 

 axis. The phenomena seen on the planet 

 depend in a great measure on the size of 

 the telescope and the magnifying power em- 

 ployed. In my work on Jupiter I have 

 habitually used a power of 890, which is 

 adapted to most conditions for seeing and 

 will show minute detail. With the same 

 telescope, using a power of 190, the appear- 

 ance of the disk is quite different, and 

 minute detail cannot be seen with distinct- 

 ness. The observers who have small tele- 

 scopes of five or six inches in aperture and 

 use a comparatively low power do not see 

 the phenomena as they would be shown by 

 larger telescopes and high power. Hence 

 in any question of disagreement, observation 

 with the small telescope should have very 

 little weight. The principle is precisely the 

 same as in the observation of double stars. 

 While a pair of close or unequal double 

 stars may be easy objects for 18^" object 

 glass, they are entirely beyond the range of 

 a 6" object glass. 



A misinterpretation of phenomena has 

 given rise to very erroneous notions regard- 

 ing the changes which take place on the 

 surface of the planet. When we look at the 

 planet Jupiter, we see only about one fifth 

 of the surface in longitude distinctly at any 

 one time, and hence in the course of two 

 hours we should have an entirely new set 

 of features under view of the eye of the ob- 

 server. The faint belts north and south 

 of the equator sometimes only extend over a 

 portion of the circumference of the planet, 

 and in such case one might see a greater 

 or less number of belts after the interval 

 of two hours or more, as has been stated by 

 Cassini and others. 



My observations during the past twenty- 



