ASTRONOMICAL PHENOMENA AND PROGRESS. 



45 



8. Mr. Trouvelot baa observed spots at leant within 

 10* of the north-polo of the BUD. 



j. Tin- tl'ivul.-nt objects sometimes seen projected 

 upon the umhrii urul penumbra of spot* ure the re- 

 in;uiiiii_' portion of the granulations composing the 

 chromosphere, more or lees dissolved by the forces 

 emanating from the interior of the photosphere. 



Temperature of the Sun's Surface. It is well 

 known that eminent authorities have differed 

 VITV widely in their estimates of the intensity 

 of solar heat. The calculations of Sir John 

 llrrsohel and Father Secchi give about 10,000,- 

 000 as the temperature at the surface ; while 

 other physicists, by applying the law of Dulong 

 and Pettit, obtain a degree of heat not great- 

 ly superior to that of a blast-furnace. Prof. 

 Langley, of the Allegheny Observatory, who 

 has for several years been devoting special 

 attention to solar physics, read a paper on this 

 subject at the October meeting of the National 

 Academy of Sciences. These recent researches 

 leave little room to doubt the approximate 

 correctness of Herschel's estimate. 



The Sun's Atmosphere. A memoir on the 

 atmospheres of the sun and planets was re- 

 cently communicated to the American Philo- 

 sophical Society, at Philadelphia, by Mr. Da- 

 vid Trowbridge, of Waterburg, N. Y. 'Assum- 

 ing the temperature of the sun's surface to 

 be 9,800,000 Fahr., and that a given volume 

 of the solar atmosphere at 82 becomes doub- 

 led at 522, the analysis of Mr. Trowbridge 

 leads to the conclusion that at the height of 

 about 2,500 miles above the sun's surface its 

 atmosphere has decreased in density to one- 

 half of what it is at the surface ; at a little 

 more than 5,000 miles, to one-fourth, and so on 

 for greater heights. Assuming further that 

 the density of the solar atmosphere at the sur- 

 face is 855,000,000,000 times what it is at the 

 outer limit, the height of the atmosphere is 

 found to be 121,000 miles, or more than one- 

 fourth of the sun's radius. This ratio is less 

 than that which actually exists in the earth's 

 atmosphere. If we now suppose the masses 

 of the terrestrial and solar atmospheres to have 

 the same ratio as the masses of the earth and 

 the sun, the density of the solar atmosphere 

 at the sun's surface will be only -fa of the den- 

 sity of the earth's atmosphere at the earth's 

 surface. The resistance offered to motion in 

 such an atmosphere is much less than it would 

 be in one of low temperature. Were the heat 

 of the sun's surface reduced to 32, the height 

 of the atmosphere under the conditions above 

 supposed would be only five miles, and at the 

 height of 665 feet the density would be re- 

 duced one-half. The pressure on a square 

 inch of surface would be 5 tons, and the den- 

 sity at the surface would be 740 times as great 

 as at the surface of the earth in other words, 

 about equal to the density of water. 



The Late Sun-spot Minimum. The observa- 

 tions of Prof. R. Wolf, Director of the Zurich 

 Observatory, indicate a minimum of solar spots 

 about the end of the year 1875. The last pre- 

 ceding minimum occurred in 1867, and the 



maximum In 1870. The mean period, accord- 

 ing to Dr. Wolf, is 11.11 years. The last 

 epoch of least disturbance took place, there- 

 fore, nearly two years before the predicted 

 time. 



Probable Existence of an Intra- Mercurial 

 Planet. It is well known that dark circular 

 spots have been seen at widely-distant inter- 

 vals passing rapidly across the solar disk. 

 These phenomena have been regarded by some 

 astronomers as transits of an unknown interior 

 planet. The fact, on the other hand, that no 

 such body has been seen during total eclipses 

 of the sun, is claimed by others as presump- 

 tive evidence that the so-called transits have 

 been duo to some other cause. But this nega- 

 tive testimony is by no means conclusive. 

 When it is remembered, moreover, that an un- 

 explained motion of Mercury's perihelion has 

 been referred by Leverrier to the disturbing 

 influence either of a planet or a zone of minute 

 asteroids within Mercury's orbit, the question 

 is certainly still open to discussion. 



The theory that this outstanding disturb- 

 ance of Mercury's motion is due, at least in 

 part, to an interior planet, has been recently 

 revived by the distinguished Director of the 

 Paris Observatory. The attention of astrono- 

 mers was called to the subject by the an- 

 nouncement that on the afternoon of April 4, 

 1876, Herr Weber, of Peckeloh, had seen a 

 small round sun-spot, which had disappeared 

 before the following morning. It was found, 

 however, that the observers at Madrid and 

 Greenwich had witnessed the same appear- 

 ance, and that their observations proved it to 

 have been a true sun-spot, not a planet. But 

 Leverrier, who had thus been led to undertake 

 a discussion of the subject, found sufficient 

 evidence informer observations to render the 

 existence of the planet highly probable. The 

 spots seen at the five following dates are re- 

 garded by the French astronomer as transits 

 of the same body : 



1802, October 10, observed by Fritsch. 

 1839, " 2, Uecuppis. 

 1849, March 12, " " Sidebotham. 

 1859, " 26, " " Lescarbault. 

 1862, u 19, " u Lummis. 



These dates are approximately satisfied by a 

 period of 33 d - O h - 82 m - 24% corresponding to 

 a mean distance of 0.201. It is remarked, 

 however, by a writer in Nature (November 2, 

 1876), that a period of 27 23 h - 8 m - 10 1 - not 

 only represents the observations selected by 

 Leverrier, but also that of Stark, October 9, 

 1819, one of the most definite on record. The 

 question is one of much interest, and as an- 

 other transit may not occur for several years, 

 diligent search will be made for the body along 

 its calculated path. If a planet of the size in- 

 dicated by the supposed transits really exist, 

 it ought certainly to be visible off the sun's 

 disk. 



New Minor Planets. The number of known 

 asteroids at the close of 1875 was 157. Eleven 



