inlets communicating with the ocean being so narrow that they cannot, in so short a time, receive or discharge sufficient water to occasion a sensible swell or subsiding of their surfaces.

Air, or the atmosphere of our earth, being specifically lighter than water, it cannot be doubted that the moon produces much higher tides in the air than in the ocean; and the effect of the moon's attraction upon our atmosphere is known to be one cause of the high winds that prevail about the time of the equinoxes, though the principal cause must be referred to the change of temperature which the atmosphere then undergoes. It remains to explain, as briefly as possible the nature of its agitation at different times, and in various parts.

When the action of the sun, the attraction of the moon, or any other cause, dilates the air in one region of the atmosphere, it must, in consequence of its expansion, force itself into another region, which, if less rare, will immediately produce a wind. Heat may be considered as the chief cause of the air's dilatation; consequently when the reflection of the solar rays from the earth has heated the air to a certain degree, it becomes lighter in those parts than in the adjacent ones, which occasions the denser air to rush into it, to restore the equilibrium

This venting of the denser air into the rarer, produces the agitation called wind; and

when the different regions of the air are very unequal, and loaded with heterogeneous particles, they rush into contact with each other, and thus give rise to the tempest which threatens the mariner with shipwreck, or to the more tremendous hurricane which tears up the firmest trees of the forest, destroys the habitations of men, and converts the fruitful land into a gloomy desart.

Whilst, however, we stand astonished at these apparent discords of nature, it is but justice to acknowledge that their effects are extremely beneficial to mankind at large, as they purify the atmosphere from all noxious vapours, dispersing all its unwholesome particles, and pouring a supply of fresh and pure air into all regions, so as to cause a proper equilibrium, and to sustain a salubrious temperature throughout the whole; at the same time that they moderate the heat of the southern regions by a stream of air from those which are colder, and meliorate the cold of the northern regions by the warm air of the south.

The former of these effects is most sensibly experienced by those who inhabit the tropics, where the solar beams fall with accumulated ardor, and cause the air to be extremely heated during the sun's influence. On the waters surrounding the tropics, however, these effects are less powerful, on account of their absorbing a greater portion of the sun's rays, when he sinks below the horizon: and

hence the air regains its proper equilibrium; for, as it has been justly remarked, "the cooler air of the ocean rushing into the heated air, refreshes and purifies it by its coolness and wholesome nitrous impregnation."

There are some parts of the torrid zone where an easterly wind prevails during the whole year, and, by its constancy affords such important advantages to merchants, and navigators in general, that it has been aptly termed the trade wind.

Various conjectures have been formed respecting the cause of this phenomenon; but the following mode of accounting for its regu lar action seems the most rational in itself, and best adapted to the plan of a work designed for the instruction of juvenile readers.

Those parts of the earth situated between the tropics are more heated than any other regions, in consequence of their receiving the greatest portion of the solar rays, and their atmosphere being considerably dilated by that circumstance. On the sun's departure, therefore, from any particular spot, the cooler air will naturally rush in, to restore the equilibrium; and this may produce the constancy of the wind under consideration.

To account for its direction we observe, that as the earth's diurnal motion is from west to east, and the apparent motion of the heavenly bodies is in an opposite direction, the sun may be said to pass westerly. Now that part of the atmosphere subjected to the sun's influ

ence, being most heated on his retreating thence in a westerly direction, the cold air rushes in behind, and occasions a constant current of wind from the east.

It is proper to remark, that the cold air west of the sun's place cannot rush into the heated air, on account of the approach of that luminary. The cool air on the north and south however, is not thus impeded, and therefore, if the sun be on either side the equator, the cross wind from that quarter combining with the easterly current, causes it to be either north-east or south-east; but the force of each is only in proportion to its stream or direction. When the sun is on the north side of the equator, the wind blows north-east; and when he is on the southern side, it blows south-east.

Our readers must be informed, however, that the easterly current of air, which we have been describing, is not constant except on the open sea; for in the vicinity of islands, &c. the heat reflected from their surfaces tends to counteract the cause of the trade winds; and from the change which the winds undergo from this circumstance, mariners give them the appellation of land winds.

It may be remarked, that in the Indian ocean there are some particular varieties in the motions of the winds, which are termed monsoons. These are periodical, blowing half the year one way, and the other half in a contrary direction.

CHAPTER X.

Of the Motion, Refraction, and Aberration of Light.

MANY and various opinions have been formed concerning the nature of light-a subject equally important in itself and interesting to every lover of science.-The Greeks appear to have considered it as an accident, or property, resulting from the first principles of things; and Des Cartes has defined it to be a globulous matter diffused through the creation, which, by the action of the sun, strikes upon our eyes, in the same manner as a rod, or staff, which if pushed at one extremity enters in the same moment at the other.

It is now well known that light is a material substance, which flows directly from the sun, as its proper fountain; and the velocity with which it moves may likewise be ascer tained with tolerable precision.-The Danish philosopher, Roemer, first demonstrated that it employs about eight minutes in travelling from the sun to our globe a distance of ninety-five millions of miles; and as this curious doctrine may naturally be expected to interest the young reader, we shall endeavour to explain it as clearly as possible; having previously remarked, that an attentive observation of the eclipses of Jupiter's satellites first led to the important discovery.