delayed in this calm belt of Cancer, that, for the want of water for their animals, they were compelled to throw a portion of them overboard.

"A vessel bound into the southern hemisphere from Europe or America, after clearing the region of variable winds and crossing the horse latitudes,' enters the north-east trades. Here the mariner finds the sky sometimes mottled with clouds, but for the most part clear. Here, too, he finds his barometer rising and falling under the ebb and flow of a regular atmospherical tide, which gives a high and low barometer every day with such regularity, that the time of day, within a few minutes, may be told by it. The rise and fall of this tide, measured by the barometer, amounts to about onetenth of an inch, and it occurs daily and everywhere between the tropics; the maximum about 10h. 30m. A.M., the minimum between 4h. and 5h. P.M.; with a second maximum and minimum about 10 P.M. and 5 A.M. The diurnal variation of the needle changes also with the turning of those invisible tides. Continuing his course toward the equinoctial line, he observes his thermometer to rise higher and higher as he approaches it; at last, entering the region of equatorial calms and rains, he feels the weather to become singularly close and oppressive; he discovers here that the elasticity of feeling which he breathed from the trade-wind air has forsaken him; he has entered the 'doldrums,' and is under the cloud-ring.'

"Escaping from this gloomy region, and entering the south-east trades beyond, his spirits revive, and he turns to his log-book to see what changes are recorded there. He is surprised to find that, notwithstanding the oppressive weather of the rainy latitudes, both his thermometer and barometer stood, while in them, lower than in the clear weather on either side of them; that just before entering and just before leaving the rainy parallels, the mercury of the thermometer and barometer invariably stands higher than it does when within them, even though they include the equator. In crossing the

equatorial doldrums, he has passed a ring of clouds that encircles the earth.'

11. Everywhere we are witnesses of the efficacy of heat in producing great changes in the aspect of nature; how enormous must be the aggregate mechanical power called into action in the bursting of the leaf-buds in one forest in spring, in the lifting of the sap into the leaflets at the summits of the trees, and in the mysterious growth and extension of the leaves and branches!

The simplest experiments of a lecture table produce evidences of electricity, arising from the most trifling friction, by the operation of heat, and by chemical action; how great, then, must the development of this gigantic agent be in the rapid growth and decay of vegetation of the torrid zone, where all the exciting causes are in constant and excessive action!

It is not wonderful, but follows as a necessary consequence for the restoration of the balance of nature, that this region should be the nursery of those fearful storms, of which modern science is happily interpreting the laws, and reducing them to so intelligible a digest, as to render it nothing short of culpable negligence in any one having to do with the navigation of a ship, who shall be found ignorant of the plain rules laid down for his guidance in the works of Redfield, Reid, Piddington, and Thom, on the storms of the Atlantic and Indian Oceans, and which have been since extended to the South Pacific by the labours of Dobson.

12. Not only is the nature of revolving storms well understood (the winds turning round the centre in the direction of the hands of a watch in the southern hemisphere, and in the contrary direction in the northern hemisphere), but indications of their approach by the falling of the barometer and the veering of the wind, are unmistakable, and warn the intelligent mariner in what direction the fearful centre of the "cyclone" lies, and which way it is travelling; and thus he is enabled to prepare for it. By his knowledge of them he may sometimes avail himself of a fair wind on its outskirts,

or he may run away from dangerous proximity to the centre of the whirl; or at the worst, if compelled to "lieto," he will know how to do this on the right tack, so as to be certain that the gale shall not veer round by the head of his ship and so take her aback.

13. The cyclones in the North Atlantic have their origin to the eastward of the West Indian Islands, in latitudes from 10° to 40° N.; they are vast whirlwinds, in which the wind blows round in a direction opposite to that of the motion of the hands of a watch. The centre moves onward in a north-westerly direction towards Florida, when its course becomes more nearly north, and then it bends towards the north-east, and follows very nearly the course of the Gulf-stream, while the circle of air involved in it is gradually extending.

In the Indian and Pacific Oceans they also have their rise within the tropics, in the Indian Ocean sweeping away from the equator and towards the west; but in the South Pacific, analogy, the most tempting substitute for reason, is disappointed, for there Mr. Dobson has shown that their track is towards the south and eastward.

This subject is of such great importance to navigation, and has yet taken so feeble a hold on the attention of seafaring men, that it has already claimed a larger space than accords with the aim and intention of this lesson, which is not that of scientific disquisition, but to direct attention to a few of the scientific aids of navigation.

LESSON III.

THE COMPASS-THE LOG-AND THE CHART. 1. In the two preceding lessons, an attempt has been made to show to the young student what aids the observant seaman may receive through a general knowledge of the natural sciences, and the advantages of going through the world with the eyes open and the attention constantly awake. We must now approach

those more exact methods which constitute, in ordinary acceptation, the sailor's professional education.

It has already been stated that the skill of the navigator consists in the utmost refinements of the ordinary habits of observation, and that the senses are to be assisted by instruments often of an extremely delicate construction. The indications which these instruments furnish are afterwards to be subjected to the crucible of mathematical reasoning, to the stern and undeviating truthfulness of geometry.

2. Geometry, indeed, lies at the root of almost every useful practical art: it recommends itself to the engineer, the astronomer, the mechanician, the navigator, to all of whom it is indispensable-and then to all who desire-and who does not!-to train and discipline their spiritual nature, to fit it to approach with understanding and rightly and justly to reason upon the marvellous works amongst which we dwell, stamped visibly, to those who will learn to see, with the seal of His knowledge who hath made all things. Dr. Whewell says "The recollection of the truths of elementary geometry has, in all ages, given a meaning and a reality to the best attempts to explain man's power of arriving at truth."

Upon the firm rock of geometrical truth have been whetted the brightest intellects whose works illumine and adorn the pages of a nation's history.

Distance, direction, and relative position, are the talismanic words of navigation, and these are essentially the property of geometry, whose subtle analysis is to be appealed to whenever they are subjects of inquiry. The standard of direction is the meridian passing through the place of the observer. The earth being supposed a sphere, the meridian is the circumference of that plane section of it which passes through these three points, viz. a pole, the centre of the sphere, and the place of the observer. It is a circle whose centre coincides with the earth's centre; it is moreover called a great circle, for none greater can be drawn on the surface of a

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sphere. Although it has been stated as a condition of the position of the circle that it passes through one pole, it must of necessity pass through the other pole also. Now, a portion of this meridian line being drawn on the surface of the terrestrial sphere, the opposite ends of it towards the northern and southern hemisphere are denominated the north and south points (fig. 4).

A line at right angles to the meridian points to the east and the west; when you face the north, the east is on your right hand and the west on your left. These two lines divide all the horizontal space by which you are encompassed, into quarters, called the four quarters of