NATURAL PHILOSOPHY. 121 



complemental or harmonic action, by which his own will be sus- 

 tained or increased or diminished, according as the amount of 

 dissimilarity existing between him and them is greater or less at 

 the time. The solar radiation proper to the same column of space 

 will be more intense in winter than in summer, and in the arctic 

 regions than in the torrid zone. 



" Again, it is a serious undei'taking to explain on the hypothe- 

 sis of universal and indiscriminate radiation, diminishing as the 

 square of the distance increases, the brightness or even the visi- 

 bility to us of the distant planets. But what we observe in natui'e 

 is precisely what we should expect on the electro-magnetic theory 

 of light. The remote planets, by being placed in jjositions which 

 would tend to involve them in coldness and darkness, are thereby 

 rendered in these respects more dissimilar to the central orb ; 

 they will therefore be all the more illuminated and warmed by 

 him; and the climate of the most remote members of our system 

 may possibly be as genial, and their day as bright, as ours. 



" Again, since all the bodies between which and the sun, ac- 

 cording to this theory, action and reaction take place, circulate 

 in planes corresponding to low latitudes in the sun, a reason 

 appears why these regions of the solar disk should be peculiarly 

 the regions of storms in his photosphere ; and the way is open 

 to a theory of sun-spots and faculfe in a direction in which indeed 

 a step has been made already by Mr. Balfour Stewart, in con- 

 necting certain states of the solar illumination with the positions 

 of the planet Venus." 



VELOCITY OF LIGHT. 



The observations of the eclipses of Jui^iter's fii-st satellite, and 

 those of the phenomena of aberration, lead directly, although 

 with a different degree of approximation, to the determination 

 of the time light occupies to run over the mean distance of the 

 sun from the earth. To deduce from this the absolute value of 

 the velocity of light referred to our ordinary units of length, we 

 must know how many miles are contained in the distance from 

 the sun to the earth. The value of this distance is found by 

 means of the parallax of the sun ; we designate thus the angle 

 under which, being at the sun's centre, we Avould see the radius 

 of the earth. The sun's parallax, calculated from the observa- 

 tions of the last transit of Venus over the disk of the sun, is fixed 

 at 8.57 seconds ; hence the distance of the sun from the earth is 

 equal to 24,109 times the radius of the earth, or to 95,384,900 

 miles. As this length is run over by the light in 8 minutes 18 

 seconds, or in 498 seconds, we conclude that the velocity of light 

 is 191,391 miles in a second. 



However, for some years, several circumstances have conspired 

 to make us believe that the determination of 8.57 seconds given 

 as the value of the sun's parallax is too small, and that th.e pai-allax 

 ought to be augmented by a quantitj^ not less than the thirtieth 

 of its value, which would elevate it to about 8.9 seconds. From 

 11 



