ASTRONOMICAL PHENOMENA AND PROGRESS. 



49 



and that no exact limit can be set to it; but he 

 has extended it to about twice the wave-length 

 previously known, and believes that beyond 

 this the heat can scarcely pass through the 

 atmosphere at all, and therefore is nearly all 

 absorbed before it can reach any human eye. 



Dr. Frolich, of Berlin, has devised apparatus 

 for the purpose of comparing the solar heat 

 from day to day with that radiated from a box 

 which was blackened with smoke and kept 

 filled with steam. His regular measures were 

 taken in and near Berlin, but he also made ob- 

 servations on the Faulhorn at the height of 

 9,000 feet. From his data Dr. Frolich found 

 that the solar heat diminished when the spots 

 were more numerous, while the higher grada- 

 tions of heat were attended with fewer sun- 

 spots. This result, although suspected by other 

 observers, has not, however, been proved. 



The Maximum of Solar Spots. The last epoch 

 of greatest sun-spots which has been investi- 

 gated occurred, according to Dr. Wolf, about 

 the end of the year 1870. From previous ob- 

 served maxima Dr. Wolf fixed the period at 

 11*1 years ; this would bring the next computed 

 maximum at the beginning of 1882. It appears, 

 however, that this maximum has been delayed 

 by one or even two years, as it was not until 

 after the middle of 1884 that the diminution of 

 the spots was so well marked as to show that 

 the maximum had actually passed. It is notice- 

 able that this epoch has not been marked by 

 auroral displays of such brilliancy as have some- 

 times been observed. The maximum of 1860 

 was most remarkable in this respect. It has 

 sometimes been supposed that brilliant auroras 

 appear at every third sun-spot maximum ; if 

 such is the case, the year 1894 may be expected 

 to be one of brilliant auroras. 



Solar Parallax and Velocity of Light No impor- 

 tant additions have been made to published 

 discussions on the subject of the solar parallax ; 

 but determinations of the velocity of light and 

 the constant of aberration have been made 

 which promise to give a more accurate value 

 of the important constant than any that can be 

 made by direct measurement. The new deter- 

 minations of the velocity of light, made first 

 by Prof. A. A. Michelson, and then by Prof. S. 

 Newcomb, agree in giving a velocity of light 

 not far from 299,860 kilometres per second. 

 The three separate results are : 



The agreement of these results shows them 

 to be sufficiently near the truth for the deter- 

 mination of the parallax. The more difficult 

 and uncertain question is that of the exact time 

 required for light to pass from the sun to the 

 earth. A new determination of the constant 

 of aberration, which depends upon this time, 

 has been made by Nyren, of Pulkowa, giving 

 the result 20"'492. The combination of this 

 VOL. xxiv. i A 



result with the above velocity of light gives 

 8'794" as the solar parallax, a result in good 

 agreement with the best recent measures. The 

 corresponding distance of the sun may be stated 

 in round numbers as 93,000,000 miles, which 

 we are now entitled to regard as the most 

 probable result, and which is not likely to be 

 altered by much more than 100,000 miles by 

 any future discoveries. The fluctuations in the 

 adopted value of this distance have been very 

 remarkable. From 1825 to 1855 the distance 

 95,000,000 miles, found by Encke, was received 

 with a degree of confidence entirely unwar- 

 ranted by the character of the observations on 

 which it depended. When it was found to be 

 in error by a larger amount than was expected, 

 it was rejected without due examination, and 

 from various observations made about 1860 the 

 distance was supposed to be about 91,000,000. 

 Subsequent investigations have shown that 

 these two extremes were almost equally far 

 from the truth, and every important step since 

 taken has been in the direction of the old value. 

 Redaction of the Transit of Venns Observations of 

 1882. Although the observations of the last 

 transit of Venus succeeded much better than 

 those of 1874, no definite information is yet 

 available respecting the progress of the reduc- 

 tions in foreign countries. In the United States 

 the work of the commission has been directed 

 principally to the measurement of the photo- 

 graphs and an examination of the instruments 

 with which the photographs were taken. The 

 measurements of the photographic plates have 

 been completed under the direction of Prof. W. 

 Harkness, U. S. N., and a careful investigation 

 of the action of the photo-heliograph under the 

 influence of the sun's ray is now in progress. 

 It is found that the reflectors change their fig- 

 ure under the influence of the sun's rays by an 

 amount which is very appreciable when the 

 most delicate measures are employed. The 

 following table shows the various stations at 

 which photographs were taken according to 

 the plan devised by the American commission, 

 and the number which are available at each : 



No. plates. 



At Washington, D. C 50 



At Cedar Keys, Fla 165 



At San Antonio, Tex 121 



At Cerro Roblero, New Mexico 216 



At Princeton, N. J 162 



At Lick Observatory, Cal 180 



At New Haven, Conn 88 



Total for northern hemisphere . 



At Wellington, South Africa 180 



At Santa Cruz, Patagonia 211 



At Santiago de Chile 19T 



At Auckland, New Zealand 51 



Total for southern hemisphere 639 



The laborious work of carrying through all 

 the calculations growing out of these measures 

 is not yet commenced, and will probably re- 

 quire several years for its completion. 



Visibility of the Satellites of Mars In 1877, when 

 the satellites of Mars were discovered, they 

 were supposed to be such difficult objects that 



