ASTRONOMICAL PROGRESS AND PHENOMENA. 



55 



at all, exceeded. It is situated at about 30 

 south latitude, and has a proper motion of its 

 own, which renders abortive any attempt to 

 compute with desired exactness, the period of 

 Jupiter's rotation. According to the deter- 

 minations of Prof. Hough, the spot has com- 

 pleted its diurnal rotations, as follows: 1879, 

 in9 h - 55 m - 34'9 9 -; 1881, in 9 h - 55 m - 37'2'- ; 1883, 

 in 9 b " 55- 38-4'- ; 1884, in 9 h - 55 m - 38'5'- ; 1885, 

 in 9 h> 55 m - 40-1'-. Prof. Young, in 1885, ob- 

 served in 50 south latitude, or 20 nearer the 

 pole than the great red spot, a white spot, 

 whose rotation period was only 9 b - 55 m - 11-14 8 -. 

 In the "Sidereal Messenger" for December, 

 1886, he says : " It is noteworthy that, although 

 this spot was in a higher latitude than the red 

 spot, it yet rotates more rapidly." This is the 

 reverse of all former experiences, and adds still 

 another to the many inexplicable phenomena 

 observed on the giant planet. The tinted spot 

 being of the same color as the belts, would be- 

 token identity of origin, but the white spots 

 must be produced by a different cause. They 

 are generally very small in appearance, and 

 sometimes many are seen simultaneously, quite 

 often in a line like a string of white beads. No 

 satisfactory reason for their formation has been 

 assigned, as also must be said of the colored 

 spots and belts. The large white spot or cloud 

 that manifested itself on the great red spot, cov- 

 ering all but a narrow annulous portion around 

 its border, and rendering it almost invisible 

 even through large telescopes, has disappeared, 

 and the spot in its original entirety may now 

 be observed, but, though as bright as in 1884, 

 yet it may easily be overlooked. 



Lunar Heat For the past two hundred years, 

 astronomers and physicists have endeavored to 

 answer the question, " Does the earth receive 

 any heat from the moon? " Prof. Forbes, in 

 1885, concentrated moonlight six thousand 

 times, without obtaining any indication of heat, 

 and he hazarded the opinion that it did not 

 amount to ^^Vsr of a degree centigrade. On 

 the other hand, Prof. Langley, with the sensi- 

 tive bolometer, concentrating upon it the moon's 

 rays by the 13-inch retractor of the Allegheny 

 Observatory has succeeded in deflecting the 

 galvanometer scale 42 divisions. His final con- 

 clusions the result of the investigations of 

 several years are thus told by himself: 

 "While we have found abundant evidence of 

 heat from the moon, every method we have 

 tried for determining the character of this heat 

 appears to us inconclusive ; and, without ques- 

 tioning that the moon radiates heat earthward 

 from its soil, we have not yet found any ex- 

 perimental means of discriminating with such 

 certainty between this and reflected heat that 

 it is not open to misinterpretation. Whether 

 we do so or not in the future, will probably de- 

 pend on our ability to measure by some process 

 which will inform us directly of the wave- 

 lengths of the heat observed." He inclines to 

 the opinion that the moon is surrounded with 

 a gaseous envelope of extreme tenuity. 



Invisible Heat Spectra. Prof. Langley, in a 

 valuable paper read before the National Acad- 

 emy, gave the results of his investigations of 

 invisible spectra, which he has shown to con- 

 tain energy heretofore unobserved, because 

 glass is opaque to them, though they will pass 

 through prisms made of rock-salt. Unfortu- 

 nately, these prisms are difficult to make, and en- 

 dure but short usage. John A. Brashear, of Al- 

 legheny, Pa., after many trials, has succeeded in 

 producing prisms of this material of great per- 

 fection, by the use of which Prof. Langley has 

 determined that, at least, the one-hundredth 

 part of the energy of the entire solar spectrum 

 comes from the ultra-violet rays. Up to 1882 

 all that was certain was wave-lengths of O00010 

 millimetre ; within two years he went down 

 to 0-00027 millimetre, and there he found that 

 the sun's effect ceased. Afterward, by the 

 use of a large Rowland grating, he made read- 

 ings down to 0-1 millimetre, when there was 

 only one vibration in 40 seconds. The short- 

 est sound-wave is 5 millimetres, and only fifty 

 times longer than the longest wave-length of 

 the spectrum according to Langley 's deter- 

 mination, which almost bridges the immense 

 gulf between light and heat on the one hand, 

 and sound on the other. 



Dark Transits of Jnpiter's Satellites. One of the 

 most inexplicable phenomena observed in the 

 solar system is the occasional transit of two of 

 Jupiter's satellites as dark objects. During their 

 transits, their sunny sides are of course present- 

 ed to us, and should be seen as brightly illu- 

 minated as the face of the planet. The third 

 and fourth satellites often make dark transits, 

 and occasionally the first is seen as a brown 

 object, but the second has never been noticed 

 otherwise than as a bright disk. Several such 

 transits of the fourth satellite, and a very few 

 of the third, have been observed during the 

 current year. 



Variable Stars. In the whole range of physi- 

 cal astronomy there is nothing more mysterious 

 than that a star, which is really a sun like our 

 own, should vary in brightness, and that peri- 

 odically, as the majority of them do. Upward 

 of 200 of this class of stars are now known, 

 several hundred others are suspected, and every 

 year new ones are observed. As it is a sub- 

 ject well adapted to amateurs (no instrument 

 save an opera-glass being required), a large 

 number of variables will, in the near future, no 

 doubt be added to the list. Ordinarily the 

 light variation is small, while in exceptional 

 cases, from being a bright star at its maximum, 

 it fades away until it becomes invisible even in 

 a powerful telescope. If the list of variable 

 stars be examined, it will appear that there are 

 but very few with periods between seventy- 

 one and one hundred and thirty-five days. It 

 will also show the connection between the pe- 

 riods and the colors of the stars, those of short 

 period being, for the most part, white, and 

 those of long period red or orange-red. The 

 most complete and valuable catalogues of vari- 



