INTRODUCTION. XXXI 



I have, myself, .always found the naked eye Letter for viewing the Zodiacal Light than telescopes. 

 Through our ship's glasses I was never able to see it at all. 



If we could have a Zodiacal Light ol'an undoubted character produced hy the full moon, not 

 only would the question before us be set at rest, but the ring would lie shown to be within the 

 orbit of the moon ; and how near we came to a case of that kind on the evening of February 14*, 

 1854, the reader will decide for himself. There was no subject connected with these observa- 

 tions, in which I was so carefully watchful ; but, in summer, the moon, when full, must rise 

 long before the crepuscule ceases, and it is only in winter months that satisfactory observations 

 of this nature can be made ; and in the few instances of this kind which offered, clouds inter- 

 fered to prevent them. 



For myself, I have no doubt that what I saw, in all the cases given in these charts, was really 

 Zodiacal I/igJut produced by the moon. When the equator and ecliptic were furthest removed 

 from each other, the Light still kept closely with the ecliptic, and, therefore, could not have been 

 atmospheric; and the boundaries, though only in one case having the altitude of the sun's 

 Zodiacal Light, still, as far as they ascended, always resembled fully those produced by the sun. 



From the deductions made in I, it is apparent that we cannot expect to get a parallax of this 

 ring; and that we can hope for only approximations to its width. In the morning observations 

 Nos. 137, 145, 15*7, 159, I appear to have got the full lateral extent northward of the Stronger 

 Light, about 30 ; and in No. 130, of the Diffuse Light 45 ; but the evening observations of 

 June, July, and August, 1853, differ somewhat from these. The inference from the whole of 

 these data would seem to be about 60 for the full width of the Stronger, and 90 for that of the 

 Diffuse Light. 



I endeavored to have simultaneous observations made in Connecticut while I was in the extreme 

 southern latitudes, but did not succeed. 



XI. 



This ring must, according to the laws of matter, rotate on its centre ; and it seems to be full of 

 commotions within itself. The existence of the pulsations, so often referred to in this book, seems 

 scarcely to admit of a doubt, recorded as they have been by observers in such distant quarters 

 of the globe. They were, as a general thing, very obscurely marked; but at times they 

 appeared to be so decided that I had no longer a doubt of their reality. They could scarcely 

 be owing to irregularities on the surface ; for the changes appear to have been too rapid and 

 extensive for such a cause. But that is possible. The following, respecting the rings of Saturn, 

 is from Laplace's Mecaniqiie Celeste: "Hence it follows that the separate rings which sur- 

 round the body of Saturn, are irregular solids, of unequal width in the different parts of their 

 circumferences ; so that their centres of gravity do not coincide with their centres of figure. 

 These centres of gravity may be considered as so many satellites, which move about the centre 

 of Saturn, at distances depending on the irregularities of the parts of each ring, and with 

 velocities of rotation equal to those of their respective rings." Bowditch's Tr., vol. v, p. 5 1C. 



If we allow an irregularity of width to the earth's ring, it may account for the changes in 

 its intensity of light; the Zodiacal Light this spring (1856) having been considered as of much 

 greater brightness than in previous years. 



s The moon was full at Greenwich February 12, 14A. 5(!m. ; allowing for the cliU'iTrmv in longitude, the observation was 

 lil. (Jh. i'Jin. after the full ; flic next evening's observation, with still more decided results, was 2d. Th. 28)K. after the full. 



