730 THE POPULAR SCIENCE MONTHLY. 



yet, owing to the imperfection of the tables, these epochs are often 

 found to be appreciably in error. There is yet another difficulty. 

 The satellites are not mere points, but, being in reality also as large 

 as or larger than our moon, they have disks of appreciable though 

 small dimensions. Accordingly, they do not vanish or reappear in- 

 stantaneously, but gradually, the process lasting in reality several 

 seconds (a longer or shorter time, according to the particular satellites 

 considered), and the estimated moment of the phenomenon thus comes 

 to depend on the power of the telescope employed, or the skill or the 

 visual powers of the observer, or the condition of the atmosphere, and 

 so on. Accordingly, very little reliance could be placed on such ob- 

 servations as a mean for determining the longitude with any consid- 

 erable degree of exactness. 



No other celestial phenomena present themselves except those 

 depending on the moon's motions. 1 All the planets, as well as the 

 sun and moon, traverse at various rates and in different paths the 

 sphere of the fixed stars. But the moon alone moves with sufficient 



1 If but one star or a few would periodically (and quite regularly) " go out " for a few 

 moments, the intervals between such vanishings being long enough to insure that one 

 would not be mistaken in point of time for the next or following one, then it would be pos- 

 sible to determine Greenwich or other reference time with great exactness. And here 

 one cannot but recognize an argument against the singular theory that the stars were in- 

 tended simply as lights to adorn our heavens and to be of use to mankind. The teleolo- 

 gists who have adopted this strange view can hardly show how the theory is consistent 

 with the fact that quite readily the stars (or a few of them) might have been so contrived 

 as to give man the means of travelling with much more security over the length and 

 breadth of his domain than is at present possible. In this connection I venture to quote 

 a passage in which Sir John Herschel has touched on the usefulness of the stars, in terms 

 which, were they not corrected by other and better-known passages in his writings, 

 might suggest that he had adopted the theory I have just mentioned : " The stars," he 

 said, in an address to the Astronomical Society, in 1827, " are landmarks of the universe ; 

 and, amid the endless and complicated fluctuations of our system, seem placed by its 

 Creator as guides and records, not merely to elevate our minds by the contemplation 

 of what is vast, but to teach us to direct our actions by reference to what is immutable 

 in his works. It is indeed hardly possible to over-appreciate their value in this point 

 of view. Every well-determined star, from the moment its place is registered, becomes 

 to the astronomer, the geographer, the navigator, the surveyor, a point of departure 

 which can never deceive or fail him the same forever and in all places, of a delicacy 

 so extreme as to be a test for every instrument yet invented by man, yet equally adapted 

 for the most ordinary purposes ; as available for regulating a town-clock as for con- 

 ducting a navy to the Indies ; as effective for mapping down the intricacies of a petty 

 barony as for adjusting the boundaries of transatlantic empires. When once its place 

 has been thoroughly ascertained, and carefully recorded, the brazen circle with which 

 the useful work was done may moulder, the marble pillar may totter on its base, and 

 the astronomer himself survive only in the gratitude of posterity ; but the record remains, 

 and transfuses all its own exactness into every determination which takes it for a 

 groundwork, giving to inferior instruments, nay, even to temporary contrivances, and 

 to the observations of a few weeks or days, all the precision attained originally at the 

 cost of so much time, labor, and expense." It is only necessary, as a corrective to the 

 erroneous ideas which might otherwise be suggested by this somewhat high-flown pas- 

 sage, to quote the following remarks from the work which represented Sir John Her- 



