ASTRONOMY. 207 



Brio-btness of a point on the sun's limb compared witli that of a i)oiut 

 near the center of the disk, 25 per cent. 



Heat received per minute from the sun upon a square metre, per- 

 pendicularly exposed to the solar radiation at the upper surface of the 

 earth's atmosphere (the solar constant), 25 calories. 



Heat radiation at the surface of the sun, per square metre per minute, 

 1,117,000 calories. 



Thickness of a shell of ice which would be melted from the surface of 

 the sun per minute, 48^ feet, or 14| meters. 



Mechanical equivalent of the solar radiation at the sun's surface, con- 

 tinuously acting, 109,000 horse-power i)er square metre; 10,000 (nearly) 

 per square foot. 



Effective temperature of the solar surface (according to Eosetti), about 

 10,0000 Cent., or 18,000° Fahr. 



SOLAR PARALLAX. 



IVIr. David Gill, Her Majesty's astronomer at the Cape of Good Hope, 

 has just issued, as a reprint from the forty-sixth volume of the " Memoirs 

 of the Eoyal Astronomical Society," his definitive paper on the deter- 

 mination of the solar parallax from observations of Mars at Ascension 

 in 1877. We think it no exaggeration to style it the most imijortant 

 separate determination of this constant which has ever been made. 

 In Mrs. Gill's charming little book, "Six Month's in Ascension: An Un- 

 scientific Account of a Scientific Expedition," the incidents and details 

 of a somewhat arduous undertaking, now become historic, were fully 

 described. The expenses of the expedition were defrayed by vote of 

 the Council of the Eoyal Astronomical Society in the first instance, 

 and later from the Government Grant Fund of the Eoyal Society. The 

 observations at Ascension were made with the heliometer owned by 

 Lord Lindsay (now the Earl of Crawford and Balcarres), in accordance 

 with what astronomers call "the method of the diurnal parallax," or 

 "east-and-west method." While much of the detail of reduction of the 

 work is presented in Mr. Gill's admirable volume, a vast deal has been 

 omitted in the printing — in full accord with the more advanced and 

 advancing notions on this subject. The original note-books and manu- 

 scripts are deposited with the society, where they may, and should, be 

 referred to, if the re-examination of the work is ever undertaken, or 

 any doubtful point arises. The final result of Mr. Gill's investigation 

 is 8".78, with a probable error of 0".012 — which gives, for the mean 

 distance of the earth from the sun, 93,080,000 miles. Most astronomers 

 will have little doubt that this value of the solar parallax is too small; 

 nevertheless, the more important of the recent researches on this sub- 

 ject show this value to be a close approximation to the truth. 



]\L Faye has just communicated to the French Academy a pai)er on 

 the actual state of our knov>'ledge of the sun's parallax. 



He considers that there is no other scientific constant, the determina- 



