nute to the difference of longitude between 151° 14′ west and 103° west.

Mere mistakes of calculation, however, matter little in such cases. They do not affect the interest of a story even in such extreme cases as in "Ivanhoe," where a full century is dropped in such sort that one of Richard the First's knights holds converse with a contemporary of the Conqueror, who, if my memory deceives me not, was Coeur de Lion's great-great-grandfather. It is a pity, however, that a novelist or indeed any writer should attempt to sketch scientific methods with which he is not familiar. No discredit can attach to any person, not an astronomer, who does not understand the astronomical processes for determining latitude and longitude, any more than to one who, not being a lawyer, is unfamiliar with the rules of Conveyancing. But when an attempt is made by a writer of fiction to give an exact description of any technical matter, it is as well to secure correctness by submitting the description to some friend acquainted with the principles of the subject. For, singularly enough, people pay much more attention to these descriptions when met with in novels than when given in textbooks of science, and they thus come to remember thoroughly well precisely what they ought to forget. I think, for instance, that it may not improbably have been some recollection of "Foul Play" which led Mr. Lockyer to make the surprising statement that longitude is determined at sea by comparing chronometer time with local time, which is found "at noon by observing, with the aid of a sextant, when the sun is at the highest point of its path." Our novelists really must not lead the student of astronomy astray in this manner.

when the altitude of the sun or a star has been noted, the seaman has learned the time of day. But to find Greenwich time is another matter; and without Greenwich time, ship time teaches nothing. as to the longitude. How is the voyager at sea or in desert places to know the exact time at Greenwich or some other fixed station? We have seen that chronometers are used for this purpose; and chronometers are now made so marvellously perfect in construction that they can be trusted to show true time within a few seconds, under ordinary conditions. But it must not be overlooked that in long voyages a chronometer, however perfect its construction, is more liable to get wrong than at a fixed station. That it is continually tossed and shaken is something; but is not the chief trial to which it is exposed. The great changes of temperature endured when a ship passes from the temperate latitudes across the torrid zone to the temperate zone again, try a chronometer far more severely than any ordinary form of motion. And then it is to be noted that a very insignificant time-error corresponds to a difference of longitude quite sufficient to occasion a serious error in the ship's estimated position. For this reason and for others, it is desirable to have some means of determining Greenwich time independently of chronometers.

This, in fact, is the famous problem for the solution of which such high rewards were offered and have been given.* It was to solve this problem that Whiston, the same who fondly imagined Newton was afraid of him, suggested the use of bombs and mortars; for which Hogarth

For the invention of the chronometer Harrison (a Yorkshire carpenter and the son of a carpenter) received twenty thousand pounds. This which would stand the test of two voyages of assigned length Harrison labored fifty years before he succeeded in meeting the required conditions.

sum had been offered for a marine chronometer

pilloried him in the celebrated madhouse scene of the Rake's Progress. Of course Whiston had perceived the essential feature of all methods intended for determining the longitude. Any signal which is recognisable, no matter by eye or ear, or in whatsoever way, at both stations, the reference station and the station whose longitude is required, must necessarily suffice to convey the time of one station to the other. The absurdity of Whiston's scheme lay in the implied supposition that any form of ordnance could propel rocket signals far enough to be seen or heard in mid-ocean. Manifestly the only signals available, when telegraphic communication is impossible, are signals in the celestial spaces; for these alone can be discerned simultaneously from widely distant parts of the earth. It has been to such signals, then, that men of science have turned for the required means of determining longitude.

Galileo was the first to point out that the satellites of Jupiter supply a series of signals which might serve to determine the longitude. When one of these bodies is eclipsed in Jupiter's shadow, or passes out of sight behind Jupiter's disc, or reappears from eclipse or occultation, the phenomenon is one which can be seen from a whole hemisphere of the earth's surface. It is as truly a signal as the appearance or disappearance of a light in ordinary night-signalling. If it can be calculated beforehand that one of these events will take place at any given hour of Greenwich time, then, from whatever spot the phenomenon is observed, it is known there that the Greenwich hour is that indicated. Theoretically this is a solution of the famous problem; and Galileo, the discoverer of Jupiter's four satellites, thought he had found the means of determining the longitude with great accuracy. Unfortunately these hopes have not been realized. At sea, indeed, except in the calmest weather, it is impossible to observe the phenomena of Jupiter's satellites, simply because the telescope cannot be directed steadily upon the planet. But even on land Jupiter's satellites afford but imperfect means of guessing at the longitude. For, at present, their motions have not been thoroughly mastered by astronomers, and though the Nautical Almanac gives the estimated epochs for the various phenomena of the four satellites, yet, ow

ing 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 discs of appreciable though small dimensions. Accordingly they do not vanish or reappear instantaneously, 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 observations as a mean for determining the longitude with any considerable degree of exactness.

No other celestial phenomena present themselves except those depending on the moon's motions.* All the planets, as well

If but a 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 possible 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 intended simply as lights to adorn our heavens who have adopted this strange view, can hardly and to be of use to mankind. The teleologists 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 amidst 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 ac tions 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 of departure which can never deceive or fail him, geographer, the navigator, the surveyor, a point -the same for ever and in all places, of a deli cacy so extreme as to be a test for every instru ment yet invented by man, yet equally adapted regulating a town-clock as for conducting a navy for the most ordinary purposes; as available for to the Indies; as effective for mapping down the

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 rapidity to act as a time-indicator for terrestrial voyagers.

It is hardly necessary to explain why rapidity of motion is important; but the following illustration may be given for the purpose. The hour-hand of a clock does in reality indicate the minute as well as the hour; yet owing to the slowness of its motion we regard the hour-hand as an unsatisfactory time indicator, and only consider it as showing what hour is in progress. So with the more slowly moving celestial bodies. They would serve well enough, at least some among them would, to show the day of the year, if we could only imagine that such information were ever required from celestial bodies. But it would be hopeless to attempt to ascertain the true time with any degree of accuracy from their motions. Now the moon really moves with considerable rapidity among the stars. She completes the circuit of the celestial sphere in 27 days (a period less than the common lunation), so that in one day she traverses about thirteen

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 passage, to quote the following remarks from the work which represented Sir John Herschel's more matured views, his wellknown "Outlines of Astronomy." "For what purpose are we to suppose such magnificent bodies scattered through the abyss of space? Surely not to illuminate our nights, which an additional moon of the thousandth part of the size of our own world would do much better; nor to sparkle as a pageant void of meaning and reality, and bewilder us among vain conjectures. Useful, it is true, they are to man as points of exact and permanent reference, but he must have studied astronomy to little purpose, who can suppose man to be the only object of his Creator's care; or, who does not see in the vast and wonderful apparatus around us, provision for other races of animated beings."

It was this doubtless which led to the distinction recognized in the book of Job, where the moon is described as "walking in brightness."

degrees, or her own diameter (which is rather more than half a degree) in about an hour. This, astronomically speaking, is very rapid motion; and as it can be detected in a few seconds, by telescopic comparison of the moon's place with that of some fixed star, it serves to show the time within a few seconds, which is precisely what is required by the seaman. Theoretically, all he has to do is to take the moon's apparent distance from a known star, and also her height and the star's height above the horizon. Thence he can calculate what would be the moon's distance from the star at the moment of observation, if the observer were at the earth's centre.

But the Nautical Almanac informs him of the precise instant of Greenwich time corresponding to this calculated distance. So he has what he requires, the true Greenwich time.

It will be manifest that all methods of finding the way at sea, except the rough processes depending on the log and compass, require that the celestial bodies, or some of them, should be seen. Hence it is that cloudy weather for any considerable length of time, occasions danger and sometimes leads to shipwreck and loss of life. Of course the captain of a ship proceeds with extreme caution when the weather has long been cloudy, especially if according to his reckoning he is drawing near shore. Then the lead comes into play, that by soundings, if possible, the approach to shore may be indicated. Then also by day and night a careful watch is kept for the signs of land. But it sometimes happens that despite of all such precautions a ship is lost; for there are conditions of weather which, occurring when a ship is nearing shore, render the most careful look-out futile. These conditions may be regarded as included among ordinary sea-risks, by which term are understood all such dangers as would leave a captain blameless if shipwreck occurred. It would be well if no ships were ever lost save from ordinary sea-risks; but unfortunately ships

are sometimes cast ashore for want of care; either in maintaining due watch as the shore is approached, or taking advantage of opportunities, which may be few and far between, for observing sun, or moon, or stars, as the voyage proceeds. It may safely be said that the greater number of avoidable shipwrecks have been occasioned by the neglect of due care in finding the way at sea.-St. Paul's.