Prof. Olmsted on the Zodiacal Light. 315 



lished in the eig 



Me moires 



Academy, together with observ^ations on the same phenomenon 

 made, at Geneva by a friend of his, M. Fatio. An elaborate di- 

 gest of these records are made by Mairan in his celebrated Trea- 

 tise on the Aurora Borealis, including also a few observations of 

 his ownj and of several other philosophers. It is interesting to 

 compare these ancient observations with such as we have been able 

 to make at corresponding times of the year; and having made 

 this comparison in numerous instances, I feel able to say that the 



zodiacal lights i?i the main^ is the same thing that it was in the 

 days of Cassini and Mairan, being subject to similar variations 

 at different seasons of the year and in different states of the at- 

 niosphere. I shall avail myself of such aid as 1 can obtain from 

 this and every other source in the remaining parts of this essay. 



NATURE AND CONSTITUTION OF THE ZODIACAL LIGHT. 



1. Length. — The extreme portions of tliis body sometimes 

 extend beyond the earth's orbit. It is obvious that, at an elon- 

 gation of 90^, it must reach a tangent drawn to the earth^s orbit 

 ^t the place of the spectator; and if it reaches beyond that tan- 

 gent, as is sometimes the case, it must of course extend beyond 

 the earth's path- According to one of our observations, on the 

 18th December, 1837, its elongation was 120^. 

 ^ The variable apparent elongation to which this phenomenon 

 Js subject is more or Jess influenced by three causes; the state of 

 the atmosphere, the inclination of its line of direction to the hori- 

 zon, and the length of the twilight. In order to eliminate the 

 effect due to atmospheric changes we require numerous series of 

 observations, continued through successive years, and, if possible, 

 instituted at long intervals of time. The mean of such an as- 

 semblage of observations would exhibit results nearly free from 

 the effects of accidental variations in the transparency of the at- 

 mosphere. Since the axis of the zodiacal h'ght does not deviate 

 far from the ecliptic, we may imagine it to be represented by a 

 portion of that circle on the artificml globe, and we shall easily 

 see that since its inclination to the horizon varies between twen- 

 ty-five and seventy-two degrees, being twenty-five at the vernal 

 equinox, (twenty-five degrees with the eastern and seventy-two 

 degrees with the western horizon,) this cause must greatly affect 

 the degree of intensity of the zodiacal light. The same must 

 obviously be the case with the variations in the length of twi- 

 light, being an hour and a half after sunset at the vernal equinox, 

 and two hours and a quarter after sunset at the summer solstice. 

 fiut were these causes, combined, the only or the chief reason 

 }vhy the apparent elongation of the zodiacal light from the sun 

 is greater at one time tlian at another, then, since at the vernal 

 equinox the elevation above the horizon is at its maximum and 



