(266.) Another species of natural signal, visible at once over a whole terrestrial hemisphere, is afforded by the eclipses of Jupiter's satellites, of which we shall speak more at large when we come to treat of those bodies. Every such eclipse is an event which possesses one great advantage in its applicability to the purpose in question, viz. that the time of its happening, at any fixed station, such as Greenwich, can be predicted from a long course of previous recorded observation and calculation thereon founded, and that this prediction is sufficiently precise and certain, to stand in the place of a corresponding observation. So that an observer at any other station wherever, who shall have observed one or more of these eclipses, and ascertained his local time, instead of waiting for a communication with Greenwich, to inform him at what moment the eclipse took place there, may use the predicted Greenwich time instead, and thence, at once, and on the spot, determine his longitude. This mode of ascertaining longitudes is, however, as will hereafter appear, not susceptible of great exactness, and should only be resorted to when others cannot be had. The nature of the observation also is such that it cannot be made at sea*; so that, however useful to the geographer, it is of no advantage to navigation.

(267.) But such phenomena as these are of only occasional occurrence; and in their intervals, and when cut off from all communication with any fixed station, it is indispensable to possess some means of determining longitudes, on which not only the geographer may rely for a knowledge of the exact position of important stations on land in remote regions, but on which the navigator can securely stake, at every instant of his adventurous course, the lives of himself and comrades, the interests of his country, and the fortunes of his employers. Such a method is afforded by LUNAR OBSERVATIONS. Though

To accomplish this is still a desideratum. Observing chairs, suspended with studious precaution for ensuring freedom of motion, have been resorted to, under the vain hope of mitigating the effect of the ship's oscillation. The opposite course seems more promising, viz. to merely deaden the motion by a somewhat stiff suspension (as by a coarse and rough cable), and by friction strings attached to weights running through loops (not pulleys) fixed in the wood-work of the vessel. At least, such means have been found by the author of singular efficacy in increasing personal comfort in the suspension of a cot.

we have not yet introduced the reader to the phenomena of the moon's motion, this will not prevent us from giving here the exposition of the principle of the lunar method; on the contrary, it will be highly advantageous to do so, since by this course we shall have to deal with the naked principle, apart from all the peculiar sources of difficulty with which the lunar theory is encumbered, but which are, in fact, completely extraneous to the principle of its application to the problem of the longitudes, which is quite elementary.

(268.) If there were in the heavens a clock furnished with a dial-plate and hands, which always marked Greenwich time, the longitude of any station would be at once determined, so soon as the local time was known, by comparing it with this clock. Now, the offices of the dial-plate and hands of a clock are these:- the former carries a set of marks upon it, whose position is known; the latter, by passing over and among these marks, informs us, by the place it holds with respect to them, what it is o'clock, or what time has elapsed since a certain moment when it stood at one particular spot. (269.) In a clock the marks on the dial-plate are uniformly distributed all around the circumference of a circle, whose centre is that on which the hands revolve with a uniform motion. But it is clear that we should, with equal certainty, though with much more trouble, tell what o'clock it were, if the marks on the dial-plate were unequally distributed,—if the hands were excentric, and their motion not uniform,— provided we knew, 1st, the exact intervals round the circle at which the hour and minute marks were placed; which would be the case if we had them all registered in a table, from the results of previous careful measurement:-2dly, if we knew the exact amount and direction of excentricity of the centre of motion of the hands;-and, 3dly, if we were fully acquainted with all the mechanism which put the hands in motion, so as to be able to say at every instant what were their velocity of movement, and so as to be able to calculate, without fear of error, HOW MUCH time should correspond to SO MUCH angular movement.

(270.) The visible surface of the starry heavens is the

dial-plate of our clock, the stars are the fixed marks distributed around its circuit, the moon is the moveable hand, which, with a motion that, superficially considered, seems uniform, but which, when carefully examined, is found to be far otherwise, and which, regulated by mechanical laws of astonishing complexity and intricacy in result, though beautifully simple in principle and design, performs a monthly circuit among them, passing visibly over and hiding, or, as it is called, occulting some, and gliding beside and between others; and whose position among them can, at any moment when it is visible, be exactly measured by the help of a sextant, just as we might measure the place of our clock-hand among the marks on its dial-plate with a pair of compasses, and thence, from the known and calculated laws of its motion, deduce the time. That the moon does so move among the stars, while the latter hold constantly, with respect to each other, the same relative position, the notice of a few nights, or even hours, will satisfy the commencing student, and this is all that at present we require.

or

(271.) There is only one circumstance wanting to make our analogy complete. Suppose the hands of our clock, instead of moving quite close to the dial-plate, were considerably elevated above, or distant in front of it. Unless, then, in viewing it, we kept our eye just in the line of their centre, we should not see them exactly thrown or projected upon their proper places on the dial. And if we were either unaware of this cause of optical change of place, this parallax· negligent in not taking it into account-we might make great mistakes in reading the time, by referring the hand to the wrong mark, or incorrectly appreciating its distance from the right. On the other hand, if we took care to note, in every case when we had occasion to observe the time, the exact position of the eye, there would be no difficulty in ascertaining and allowing for the precise influence of this cause of apparent displacement. Now, this is just what obtains with the apparent motion of the moon among the The former (as will appear) is comparatively near to the earth the latter immensely distant; and in consequence

stars.

of our not occupying the centre of the earth, but being carried about on its surface, and constantly changing place, there arises a parallax, which displaces the moon apparently among the stars, and must be allowed for before we can tell the true place she would occupy if seen from the centre.

(272.) Such a clock as we have described might, no doubt, be considered a very bad one; but if it were our only one, and if incalculable interests were at stake on a perfect knowledge of time, we should justly regard it as most precious, and think no pains ill bestowed in studying the laws of its movements, or in facilitating the means of reading it correctly. Such, in the parallel we are drawing, is the lunar theory, whose object is to reduce to regularity, the indications of this strangely irregular-going clock, to enable us to predict, long beforehand, and with absolute certainty, whereabouts among the stars, at every hour, minute, and second, in every day of every year, in Greenwich local time, the moon would be seen from the earth's centre, and will be seen from every accessible point of its surface; and such is the lunar method of longitudes. The moon's apparent angular distance from all those principal and conspicuous stars which lie in its course, as seen from the earth's centre, are computed and tabulated with the utmost care and precision in almanacks published under national control. No sooner does an observer, in any part of the globe, at sea or on land, measure its actual distance from any one of those standard stars (whose places in the heavens have been ascertained for the purpose with the most anxious solicitude), than he has, in fact, performed that comparison of his local time with the local times of every observatory in the world, which enables him to ascertain his difference of longitude from one or all of them.

(273.) The latitudes and longitudes of any number of points on the earth's surface may be ascertained by the methods above described; and by thus laying down a sufficient number of principal points, and filling in the intermediate spaces by local surveys, might maps of countries be constructed. In practice, however, it is found simpler and easier to divide each particular nation into a series of great triangles, the angles of

which are stations conspicuously visible from each other. Of these triangles, the angles only are measured by means of the theodolite, with the exception of one side only of one triangle, which is called a base, and which is measured with every refinement which ingenuity can devise or expense command. This base is of moderate extent, rarely surpassing six or seven miles, and purposely selected in a perfectly horizontal plane, otherwise conveniently adapted to the purposes of measurement. Its length between its two extreme points (which are dots on plates of gold or platina let into massive blocks of stone, and which are, or at least ought to be, in all cases preserved with almost religious care, as monumental records of the highest importance), is then measured, with every precaution to ensure precision, and its position with respect to the meridian, as well as the geographical positions of its extremities, carefully ascertained.

(274.) The annexed figure represents such a chain of

K

triangles. A B is the base, O, C, stations visible from both its extremities (one of which, O, we will suppose to be a national observatory, with which it is a principal object that the base should be as closely and immediately connected as possible); and D, E, F, G, H, K, other stations, remarkable points in the country, by whose connection its whole surface may be covered, as it were, with a network of triangles. Now, it is evident that the angles of the triangle A, B, C being observed, and one of its sides, A B, measured, the other two sides, A C, B C, may be calculated by the rules of trigonometry; and thus each of the sides A C and B C

The greatest possible error in the Irish base of between seven and eight miles, near Londonderry, is supposed not to exceed two inches.