METEORIC STONES AND SHOOTING STARS. 



The semi-diameter c A of this lenticular mass is nearly equal to 

 that of the earth's orbit, so that at certain times the earth 

 grazes its edges at A and A', and probably may pass through a 

 portion of the nebulous matter. 



24. If this matter consist to any extent of solid masses of 

 small dimensions, they may pass through the earth's atmo- 

 sphere, on these occasions producing the phenomena of shooting- 

 stars, or they may strike the surface or be drawn down upon it 

 by terrestrial gravitation, and produce the phenomena of 

 meteoric stones. 



The appearance of the zodiacal light is easily explained. Let 

 H o represent the line in which the plane of the horizon 

 intersects the lenticular nebula after sunset. In that case, 

 o A' H will be below, and o A H above the horizon. The matter 

 composing o A H being illuminated by the sun will, so far as it 

 may have reflective power, be visible. In fact it is seen in 

 certain positions of the earth in relation to the sun. It presents 

 the appearance of a faint and ill-defined comet, and is usually 

 seen soo.n after sunset about the months of March, April, and 

 May, and before sunrise about the months of September, 

 October, and November. It appears as a luminous cone 

 extending from the horizon obliquely upwards as has been 

 already stated in the direction of the solar equator, and 

 therefore nearly in that of the ecliptic, or the zodiac, and 

 hence has been called " zodiacal light." The semi-diameter A c 

 subtends an angle at the earth, which varies with the position 

 of the earth from 40 to 90. In some cases, therefore, the 

 vertex A is near the zenith, when the sun is below the horizon. 

 The breadth B B', of the base of the sun subtends an angle 

 which varies from 8 to 30. 



The zodiacal light is very faint and ill-defined when seen 

 in the higher latitudes, but is much brighter and clearer 

 within the tropics. 



The matter composing this nebulous envelope of the sun 

 may, according to Sir J. Herschel, be conjectured to be no other 

 than the denser part of that medium, which, we have some 

 reason to believe, resists the* motion of the comets, loaded 

 perhaps with the actual materials of the tails of millions of 

 those bodies, of which they have been stripped in their suc- 

 cessive visits to the sun. An atmosphere of the sun the zodiacal 

 light cannot be, in any proper sense of that term, since the 

 existence of a gaseous envelope propagating pressure from part 

 to part, subject to mutual friction in its strata, and therefore 

 rotating in the same or nearly the same time with the central 

 body, and of such dimensions, and ellipticity, is utterly incom- 

 158 



