244 American Association^ 



tbe officers of the watch to look out for the light, and when the}' 

 saw it to call the commander and such other officers as took an in- 

 terest in its observation. These gentlemen made diagrams of the 

 appearance, which were handed to him for comparison, so that the 

 results vrere not tliose of the observation of one but of many ob- 

 servers. At first few could distinguish this light, but after some 

 time, they could readily point it out in its relation to the stars. 

 In these observations he Avas much indebted to Mr. Dana. The 

 first time of passing through the tropics, where the light is most 

 visible, it w^as unfortunate that there was great obscurity of the 

 atmosphere. This caused a want of means for comparison ; but 

 this deficiency had been supplied by some obtained from the Japan 

 expedition. All the observations of the Zodiacal light showed 

 that it had not changed its appearance since two centuries ago 

 when first noticed by Cassini, and by observing it particularly, 

 with reference to the great circles of the globe — ecliptic and equi- 

 noctial — it becomes manifest that all changes of its appearances 

 depend on the position of the spectator on the surface of the r/?o5f. 

 After mentioning the several theories which have been promul- 

 gated at different times to account for this phenomenon, he stated 

 his opinion that none of them satisfactorily accounted for the 

 observed facts. The drawings were made as the phenomena ap- 

 peared to the eye, being projected therefore on vertical and hori- 

 zontal lines. When the light first appeared it was generally as an 

 arc near the horizon ; but it rose in a few minutes from 

 30° to 80°. The light then began to spread, and show a dif- 

 fused light, Avhich gradually became more visible, till it Avas in its 

 perfection at the moment Avheu the arc obtained its highest alti- 

 tude, and it became difficult to ascertain where the diff'used light 

 began and ended. At length it lessened in intensity and 

 the w^hole gradually subsided. The apex of the light 

 was always East or West of the Sun, usually about 90°, 

 sometimes 100°. The evening and the morning zodiacal light did 

 not agree in phase or azimuth. In fact it Avas plain that the cause 

 of this light could not be far removed from the earth's atmosphere. 

 Within the tropics, and Avhcn the ecliptic Avas perpendicular to 

 the horizon, the zodiacal light Avas confined to a slender column, 

 having its diffused light little extended. Without the tropics it 

 Avas alwaj's inclined to the horizon. Corresponding observa- 

 tions made on the same day also shoAved an inclination in op- 

 posite directions, the two appearances of the light being inclined 



