ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 513 



differences of thickness of the stratum of air through which the heav- 

 enly bodies are viewed must be considerably greater at spring tides 

 than at the opposite phases. 



THE ZODIACAL LIGHT. 



The zodiacal light still remains very much a mystery. It may be a 

 reflection, by a multitude of exceedingly small and light solid particles 

 driven off from the sun, of the solar beams, and, indeed, it seems highly 

 probable that the development of electricity in the chromosphere may 

 be sufficient to propel small particles with much greater force away 

 from the sun than gravitatiou can exercise in restraining them. When 

 the surface is large compared with the mass, as in the smallest particles 

 larger than molecules, the electric forces need not be disproportionately 

 great to exceed by many times the force of gravitation even of the sun. 

 If the interplanetary spaces be rilled with reflecting and nonreflecting 

 motes derived from sun, and moving at a speed much exceediug that of 

 aerolites, we must suppose thatour atmosphere is always receiving within 

 its borders multitudes of these particles which are instantly consumed 

 by friction. Moreover, if such emission proceeds continually from the 

 sun, a similar process takes place from the more distant stars, and the 

 whole of recognized space is traversed by small elementary particles 

 traveling at an enormous speed. The phenomena of the tails of comets 

 tend to corroborate this opinion. In fact, considering the immense num- 

 ber of comets in space, it seems impossible that such small particles 

 can be absent. Compared with their extension, their united mass may 

 be very small indeed within the orbits of the planets. Like meteor 

 swarms, they do not apparently affect the motion of comets or of plan- 

 ets. None the less, the part they fill in the economy of the universe 

 may be considerable. 



HEIGHT OF THE ATMOSPHERE. 



Meteors which have been calculated to pass with ignition through 

 air at a height sometimes as great as 300 miles; aurorse, of which the 

 height has been estimated by careful observation sometimes to exceed 

 281 miles; and the duration of twilight, with polarizing effects of the 

 sky, giving a height of 198 to 212 miles, agree in showing a much greater 

 altitude for the extension of our atmosphere than was formerly supposed. 

 First 5 and then 45 miles was generally stated as the outside limit. And 

 we have to remember that at this great altitude of about 300 miles the 

 atmosphere is dense enough to produce very palpable effects. It would 

 be a bold proposition to assign a limit to the atmosphere within 1,000 

 miles. 



ATMOSPHERIC DUST AND THE REFLECTION OF LIGHT. 



Atmospheric dust, or particles large enough to arrest the movement 

 of light waves, exercise a very important function in the illumination 



