70 



ASTRONOMICAL PHENOMENA AND PROGRESS. 



to time, they claim to have found that when 

 Venus and Jupiter cross the solar equator, the 

 solar activity is more confined to the equa- 

 torial regions of the sun, and that when those 

 planets are farthest removed from the solar 

 equator, the activity extends outward toward 

 the solar poles. In the tables which they pre- 

 sent, they point out how closely the minor 

 epochs of solar activity in their approach to the 

 equator agree with the epochs at which Yenus 

 crosses the solar equator, and how the solar 

 activity spreads out toward the poles at 

 those times when Venus is farthest removed 

 from the equator. Lastly, they announce the 

 conclusion that solar activity, as shown in the 

 phenomena of sun-spots, would not exist but 

 for planetary motion, any more than certain 

 physical phenomena of the planets would be 

 produced without solar influence. 



The Solar Parallax. Mr. Stone (Greenwich 

 Observatory) has detected a small error in 

 Leverrier's determination of the solar parallax 

 in the numerical work. The earth has an 

 orbital motion around the common centre of 

 gravity of the moon and the earth, the diam- 

 eter of this orbit being about 6,000 miles. In 

 Leverreir's method, the earth's motion in this 

 small orbit is taken advantage of to estimate 

 the sun's distance. The size of the subsidiary 

 orbit being determined from the estimated ' 

 mass of the moon, and the displacement of 

 the sun due to the earth's excursions in her 

 monthly orbit being determined from a care- 

 ful examination of a long series of observations, 

 the ratio of the sun's distance from the moon 

 is then ascertained by a simple calculation. 

 Owing to a mistake in the numerical work, 

 Leverrier took the moon's mass at -^^ in- 

 stead of srV? f the earth. The effect of the 

 correction is to reduce the solar parallax from 

 8". 95 to8".91, corresponding to an increase of 

 over 400,000 miles in the sun's estimated dis- 

 tance. The weak point of the method clearly 

 lies in the great variation resulting from a very 

 small change in the estimated value of the 

 moon's mass. 



The corrected estimate of the sun's distance 

 by Leverrier's method agrees with Hanseii's 

 determination from the moon's parallactic in- 

 equality. Mr. Stone, who had obtained the 

 value 8".94 for the solar parallax from observa- 

 tions of Mars, has lately deduced the value 8". 85 

 (with a possible error of 0".056) from the Green- 

 wich lunar observations made near the epoch 

 of maximum lunar parallactic inequality. 

 Quarterly Journal of Science, No. XV. 



The Chemical Intensity of Total Daylight. 

 Henry E. Eoscoe communicated to the Royal 

 Society, at its June meeting, a paper on " The 

 Chemical Intensity of Total Daylight at Kew 

 and Para in 1865-'67." The paper contains 

 the results of a regular series of measure- 

 ments. In the Kew observations, the first re- 

 sult noted was this, that the mean chemical in- 

 tensity for hours equidistant from noon is con- 

 stant that is, it is equal for equal altitudes of 



the sun ; thus, the mean of all the observations 

 made about 9 h 30 m A N M. corresponds with the 

 mean at 2 h 30 m p. M. 



Mean of Times Mean of Chem. 



of Observ. Intensity. 



Mean of 552 morning ob- ) 07 ... 



nervations in 186P67J 1 9A> 41m " - 10 



Mean of 529 afternoon | 



observ. in 1865-'67. . . 



0.107 



Hence, the author concludes that when the 

 disturbing causes of variation in amount of 

 cloud, etc., are fully eliminated by a sufficient 

 number of observations, the daily maximum of 

 chemical intensity corresponds to the maxi- 

 mum of the sun's altitude. He then shows, 

 from measurements made at varying altitudes 

 of the sun at Heidelberg and Para, that the 

 relation between sun's altitude and chemical 

 intensity may be represented by the equation 

 CI B =CI xconst. a, where CI a represents the 

 chemical intensity at a given altitude (a) in cir- 

 cular measure, CI the chemical intensity at 

 the altitude O, and const, (a) a number to be 

 calculated from the observations. The agree- 

 ment of the chemical intensities, as actually 

 found at Heidelberg, and the calculated result 

 are seen in the following table : 



A similar relation is found to hold good for 

 the Para observations. Curves exhibiting 

 the daily rise and fall for each of the 24 

 months from April, 1865, to March, 1867, in- 

 clusive, accompany the paper. The curve of 

 yearly chemical intensity is found to be un- 

 symmetrical about the vernal and autumnal 

 equinoxes ; thus in spring and summer the re- 

 sults are as follows : 



1865 and 186T. Mean Ch. Int. 



March, 1867 30.5 



April, 1865 97.8 



September, 1865.. 107.8 



August, 1865 88.9 



1866. Mean Ch. Int. 



March 34.5 



April 52.4 



September 70.1 



August 94.5 



Or for 100 chemically active rays falling 

 during the months of March and April, 1865, 

 1866, and 1867, at Kew, there fell, in the cor- 

 responding autumn months, 167 rays, the 

 sun's mean altitude being the same. The 

 yearly integral for the 12 months, January to 

 March, 1867, and April to December, 1865, is 

 55.7, w r hereas that for the twelve months of the 

 year 1866 is 54.7. 



Most of the knowledge that we possess of 

 the distribution and intensity of the chemically 

 active rays in the tropics is derived from the 

 statements of photographers. It has been said 

 that the difficulty of obtaining a good photo- 

 graph increases as we approach the equator. 

 Thus in Mexico, where the light is very in- 



