spring tides will be greater than in summer, when he is farther off; and the neap tides, on that account, will be less. And, for a like reason, as the moon moves in an elliptical orbit round the earth, and is nearer to us at some times than at others, the tides will, at those times, be greater, and at the opposite points of her orbit less. Some variations, likewise, take place in consequence of the different declinations of the sun and moon at different times. For if either of these luminaries were at the pole, it would occasion a constant elevation both there and at the opposite one, and a constant depression at the equator; so that as the sun and moon gradually decline from the equator, they lose their effect, and the tides become less; and when they are both in the equator, the tides of course become greater.

These are the principal phænomena of the tides; and where no local circumstances interfere, the theory and facts will be found to agree. But it must be observed, that what has been here said, relates only to such places as lie open to large oceans. In seas and channels, which are more confined, a number of causes concur, which occasion considerable deviations from the general rule, Thus, it is high water at Plymouth about the sixth hour; at the Isle of Wight about the ninth hour; and at London-bridge about the fifteenth hour, after the moon has passed the meridian. And at Batsha, in the kingdom of Tonquin, the sea ebbs and flows but once a day; the time of high water being at the setting of the moon, and the time of

low water at her rising. There are, also, great variations in the height of the tides, according to the situation of coasts, or the nature of the streights which they have to pass through. Thus, the Mediterranean and Baltic seas have very small elevations; while, at the port of Bristol, the height is sometimes forty feet; and at St. Malo's it is said to be near a hundred.

What has been said of the ocean may likewise be applied to the air; for the surface of the atmosphere being nearer to the moon than the surface of the sea, it is plain that the aerial tides must be much more considerable than those of the ocean: and on this account it should seem to follow, that the mercury in the barometer would sink considerably lower than at other times, when the moon passes the meridian; because her action on the particles of air, must, at that time, make them much lighter. But it must be considered, that in proportion as these particles are rendered lighter, a greater number of them are accumulated, till the deficiency of gravity is made up by the height of the column; and as there is then an equilibrium, the pressure will evidently be the same as before; and consequently the mercury in the barometer, cannot, in any respect, be affected, by means of these tides.

LETTER IX.

OF THE LATITUDE AND LONGITUDE, AND THE METHODS OF DISCOVERING THEM.

IN almost all difficulties we have our resources; and such are the inventive powers of the mind, that there are but few things, which it is useful for us to know, but what some means have been devised for obtaining a knowledge of them. To determine the true place of a ship at sea, is a problem, which was formerly placed in the same degree of probability with the secret of prolonging life, the perpetual motion, the squaring of the circle, and other similar projects: but, impossible as the solution of it might appear, it can now be obtained to a degree of precision, sufficient to answer most practical purposes. By the help of a few books, and a quadrant, the mariner can not only inform himself of the situation he is in, but also how far he has travelled, what distance he has to go, and how he must direct his course to arrive at the place he designs to visit.

In this enquiry, nothing more is necessary, than to find the latitude and longitude of the places under consideration; for these being once known, by only turning to a common map or globe, the places themselves will be found by inspection. What is to be understood by these terms, and how they are applicable to the purposes mentioned, you will, no doubt, be anxious to enquire; and I shall ́endeavour to satisfy your curiosity to the utmost

K

of my power. The subject is no less useful than curious, being the very foundation both of geography and navigation; and, in order that you may obtain as clear an idea of it as possible, we will begin with the first principles, and proceed gradually, step by step, to their application.

In the first place, then, as it is absolutely necessary for you to have a perfect conception of what is meant by the poles of the earth, the equator, and the meridians, I shall describe them in as familiar a manner as the subject will admit, and at the same time show you their use and design. The poles are the two extremities of the earth's axis; or those points where the imaginary line, round which it performs its daily revolutions, meets the earth's surface that which is directed towards the most northern point of the heavens, being called the north pole; and that which is directed towards the most southern point, the south pole. So that they are diametrically opposite to each other, and always preserve the same relative situation.

"Two poles turn round the globe, one seen to rise
O'er Scythian hills, and one in Libyan skies;
The first sublime in heav'n, the last is hurl'd
Below the regions of the nether world;
Where, as they say, perpetual night is found,
In silence brooding on the unhappy ground:
Or, when Aurora 'leaves our northern sphere,
She lights the downward heav'n, and rises there;
And when on us she breathes the living light,
Red Vesper kindles there the tapers of the night.",

VIRGIL.

It is, also, to be observed, that these two points have not been arbitrarily assumed by geographers

and astronomers, to answer their particular purposes, for they are pointed out to us by the nature and constitution of the globe, and are easily distinguished from all others. The nearer we approach to them, the more we find the earth becomes barren and inhospitable; so that, under the poles, the cold is so excessive, that the country is, in all probability, wholly uninhabitable. Many attempts have been made to explore these remote regions, but they have all hitherto failed; and there is great reason to imagine that they are utterly inaccessible (f). The immense quantities of ice with which the ocean, in those parts, is continually covered, renders navigation impracticable; and any method of approaching them by land, would be subject to still greater impediments.

From these considerations, you will be able to obtain a precise and determinate idea of the poles. Imagine now a circle to be drawn round the globe, exactly in the middle between these two points, and this will be the equator; which, properly speaking, is a great circle of the earth, that separates the northern from the southern hemisphere, and is every where at an equal distance from the two poles. This circle is also no less remarkable, on account of its situation, than the poles themselves; the heat here being almost as intense as the cold is there; for which reason, the ancients imagined the

(ƒ) The nearest approach towards the north pole was that made by Captain Phipps, in the year 1773, when he penetrated as far as latitude eighty degrees thirty-seven minutes; where the sea was entirely blocked up with ice.