ASTRONOMY. 221 



seem natural for science to reach, namely, that drawings are only valu- 

 able when made by strict micrometrical measurements, aud, indeed, that 

 drawiugs on anj' other basis are misleading. He overlooks the fact how- 

 ever that a single central transit of the spot over a wire is several times 

 more accurate than a micrometer measurement taken when the spot i- 

 Dot central. Jupiter presents such a variety of phenomena on his disk 

 at different times that it has been the fashion to suppose that his sur- 

 face is subject to sudden and rapid changes. The observations of the 

 professor do not confirm this beUef. "On the contrary," he says, "all 

 minor changes in the markings or spots have been slow and gradual, 

 such as might be produced by measurable mechanical forces. In fact, 

 the principal features have been permanent, no material change being 

 detected by micrometer measurement." 



From 1,379 micrometer measurements on the great red spot, on the 

 equatorial belt, the equatorial white spots, and the polar spots, a variety 

 of interesting data are presented. Computed from observation of the 

 red spot in 1879 tha rotation period of Jupiter is 9^. 55™. 34*., or 8 sec- 

 onds greater than the previously accepted value. In 1880 this value 

 was 9^\ 55™. 35.2^ Computed by the polar spots the rotation period 

 is 9^. 55"*. 35.1^ The computation from the equatorial white spots 

 shows that these spots are in motion on the surface of the planet, the 

 drift being- about 270 miles per hour in the direction of the planet's 

 rotation, or, in other words, that they made a complete revolution arouud 

 the planet in about forty-two days. 



Professor Hough regards the red spot as fixed. The equatorial belts 

 he also seems to regard as fixed. The generally-accepted theory is that 

 this planet is enveloped in a dense atmosphere; that the belts are a solid 

 portion of the planet, and that the minor spots are clouds floating in 

 the atmosphere. Professor Ilough suggests an hj pothesis, namely : 

 that "the stirface of the planet is covered with a liquid semi-incandes- 

 cent mass ; that the belts, the great red spot, and other markings are 

 composed of matter at a lower temperature. The egg-shaped polar 

 white spots are openings in the semi-fluid crust." This hyi)othesis, he 

 thinks, would account for the slow and gradual changes, which do not 

 seem reasonable on the simple atmosi)heric theory. 



The great red spot on Jupiter's disk has for two or three years been 

 attracting the attention of astronomers, and has been the subject of 

 almost endless observations. Dr. Jedrzejewicz has published some in- 

 ferences from his observations for ascertaining the time of rotation of 

 the eastern extremity of the spot, made at his private observatory at 

 Plonsk, during the winter of 1880-'81. The instrument employed is a 

 refractor, six-inches aperture, with powers 225 to 300. In December 

 he measured the length of the spot 9".S, and considers that his own 

 observations, compared with those of Professor Schmidt at Athens, 

 indicate that the length of the spot remained unchanged during the 

 winter. On this assumx)tion he finds for the time of rotation 9''. oo'^. 



