SCENES ON THE PLANET MERCURY. 67 



prise almost exactly eighty-four terrestrial days. According as 

 the arc of solar oscillation is all above the horizon of the observer 

 or all below it, or partly above and partly below it, there will be 

 different appearances and a different distribution of light and 

 heat. In the regions, covering three eighths of the planet, where 

 the arc is all below the horizon, the sun will never be seen, and 

 the darkness will be perpetual. Thick and eternal night will 

 reign there, except perhaps from the accidental appearance of 

 some light produced by refraction and atmospheric glows, or phe- 

 nomena like the aurora borealis ; together with the light emitted 

 by the stars and planets. 



Another part of Mercury, including also three eighths of its 

 surface, will have the arc of oscillation all above its horizon, and 

 will be continually exposed to the rays of the sun, without any 

 other change than the variations in the obliquity of the rays 

 through the different phases assumed during the period of eighty- 

 eight days. Night is absolutely impossible. In other regions, cov- 

 ering a quarter of the planet, in which the arc of oscillation is 

 partly above and partly below the horizon, there will be alterna- 

 tions of light and darkness. In these privileged regions the pe- 

 riod of eighty-eight days will be divided into two intervals, one 

 characterized by a continuous light, the other by darkness ; the 

 two intervals will be equal in some places, of different length in 

 others, according to the position of the place on the surface of the 

 planet, and the length of the part of the solar arc which appears 

 above the horizon. 



The possibility of organic life in a planet constituted after this 

 manner depends on the existence of an atmosphere capable of dis- 

 tributing heat into different regions, in such a way as to diminish 

 the extremes of heat and cold. Schroeter, a hundred years ago, 

 suspected the existence of an atmosphere round Mercury; my 

 observations afford more definite indications of it, and affirm its 

 existence with a much greater probability. The spots of the 

 planet are most clearly visible when they are in the central parts 

 of the disk, and grow dimmer and ultimately disappear as they 

 approach the border. I have been able to assure myself that this 

 phenomenon is not merely due to the greater obliquity of the per- 

 spective, but is because some obstacle is really presented to the 

 view of spots situated in such positions. That obstacle can hardly 

 be anything else than the greater extent of atmosphere that the 

 light-rays have to traverse in coming from the edges than from 

 the center of the disk. We have, therefore, reasons for believing 

 that the atmosphere of Mercury is less transparent than that of 

 Mars, and more nearly like that of the earth. The circular contour 

 of the planet, moreover, in which the spots become less visible, 

 always appears more luminous than the rest, but often irregularly 



