INTRODUCTION. XV 



the succeeding j r ear. It is probable that, in the former case, the Light extended further up than 

 is recorded perhaps as far as in the latter case; but my eye then not being so accustomed to 

 trace tin- dimmer markings of its higher portions on the sky, I failed to observe them. Some 

 morning observations which I have made since my return home in August, 1855, confirm the 

 higher altitudes. I was also, at first, not sufficiently careful in getting the exact curve of the 

 effulgent Light (see Nos. 197, 199, &c., compared with 313 and 323, &c.), having been con- 

 tented simply with noting the altitude of that peculiar brightness. 



It will be seen that there is a full line with dots beneath, towards the upper part of most of 

 the charts. These lines represent the course of the spectator's zenith for the hours given, and 

 are intended to show his change of place as regards the ecliptic ; so that we may know whether 

 such changes have any connexion with the changes in the Zodiacal Light. They seem to have 

 a very close and pretty uniform connexion. We must bear in mind, however, that these lines 

 are his course projected on the sky, and that his real change of place is of infinitesimal amount 

 compared with what we see it in the chart. Where there is but one observation, his zenith is 

 represented by a dot with a circle around it. If his zeniths are beyond the limits of the chart, 

 they are given in the opposite records. When the sun also could not be entered on the chart, 

 his longitude is given on the opposite page. 



In addition to this remark about the greater reliability of the latter portion of these observa- 

 tions, I would caution the reader against drawing conclusions from isolated data, if contrary to 

 the general mass. In a matter so indefinite as the Zodiacal Light, and where mistakes are so 

 easy, I was constantly liable to be misled ; yet where appearances seemed to differ from those of 

 the ordinary character, I never considered myself at liberty to do otherwise than to give them 

 with their incongruities. But still, as already remarked, I should consider it dangerous to 

 draw conclusions from such, or, at present, from any other than the general facts. 



My opportunities for observation, it will be seen from these records, were very good. Of the 

 seven hundred and fifty days included in my observations, three hundred and twenty-eight were 

 spent within the tropics; and, in the balance of the time, our cruising extended from 41 49' 

 N., to 53 28' in the south. The light always appears to best advantage near the equator; but 

 we shall see, also, that the changes in latitude, from high north to high south, were of the 

 iitrnost consequence, as regards results. 



Whenever new phenomena, which were constantly occurring, threw me into doubt, and made 

 me distrustful of my own eyes, or needing confirmation of their correctness, I could always find 

 help among the night-watchers on deck; and also a choice of help, for sometimes I needed the 

 aid of intelligent persons ; and sometimes of others, who, without knowing what was meant, 

 could only look, and tell me what they saw. My state-room was also on the quarter-deck, and 

 within ten feet of the sentry at the cabin-door, who could wake me at any mimite, for which 

 orders had been left. 



But though frequently calling in the aid of other eyes, as just described, and with very valu- 

 able results, still the observations here given are all my own ; for I was satisfied at the outset 

 of the importance of one pair of eyes and of one judgment, to see and decide in every case. And 

 thus, although for six consecutive months, so sick as often to be unable to walk or stand without 

 support, I still kept to my work; and the result, whatever it may be worth, has the merit of one 

 uniform judgment trained by some experience, and stimulated, I know, by deep earnestness in 

 the cause. 



