1886.] NEW YORK ACADEMY OF SCIENCES. 239 



have deduced values of the paralhix all the way from 8". 6 to 

 8". 95, corresponding to a distance ranging from 95,000,000 to 

 91,500,000 miles. 



The case is quite different with the heliometer observations of 

 the opposition of Mars, in 1877, made ])y Mr. Gill, at Ascension 

 Island. These give in a most definite and apparently authorita- 

 tive manner a value of 8". 783, and are apparently irreconcila- 

 ble witii any value much greater than 8". 81, or less than 8". 75. 

 So far as can be judged from the number, nature, and accord- 

 ance of the observations, I believe we must accept this as the 

 most trustworthy of the geometrical metliods yet employed; 

 though the weight of the result would certainly be increased if 

 it did not depend to such an extent upon the work of a single 

 individual. 



The confidence of astronomers in the correctness of this value 

 is greatly fortified by the fact that the most recent and reliable 

 determinations of the velocity of light, made by Michelson and 

 Nevvcomb, in 1877, 1S80, 1881, and 1882, when combined with 

 tlie Pulkowa constant of aberration determined by Nyren from 

 all the data available up to 1882, give a sohar parallax accordant 

 with the preceding, almost to the hundredth of a second — 

 8". 794 as against 8". 783. It is true there are possible theoreti- 

 cal objections to the method; as, for instance, that the result 

 may be slightly affected by the motion of the solar system 

 through space. Enough is not known certainly about the con- 

 stitution of the medium that transmits light tlirougli space, to 

 decide all such questions a 2Jriori and autlioritatively; but it is 

 unquestionable that any correction needed on account of such 

 possible causes of error must be very minute. 



We believe, therefore, that it is safe to assume pretty con- 

 fidently, that the solar parallax is about 8". 8 (though probably a 

 trifle less), which makes the sun's mean distance 93,000,000 

 miles, with an error not likely much to exceed 150,000 miles. A 

 larger value of the parallax (about 8". 85) still holds its ground 

 in the nautical almanacs, and undeniably is nearer the average ot 

 the results given by all the known methods. But none of the other 

 methods seem to us to compete at all in precision with the two 

 whose authority we accept. 



T7ie Sun and Meteorology. 



The study of the solar surface has been carried on very persist- 

 ently by Spoerer, in Germany, as well as by others, and a great 

 amount of material has been collected bearing upon the theory 

 and nature of sun-spots, and their periodicity. The extensive 

 series of photographs obtained at Kew, and at Dehra Doon, in 

 India, constitutes almost a continuous record of the solar surface 



