a little to the north point of the compass, and this the more so as the place is situated farther towards the north; on the south side the wind will be southerly.

Charles. The greater part of this tract of the globe is water; and I have heard you say, that transparent mediums do not receive heat from the sun.

Father. The greater part is certainly water: but the proportion of land is not small almost the whole continent of Africa, a great part of Arabia, Persia, the East-Indies, and China, besides the whole nearly of New Holland, and numerous islands in the Indian and Pacific oceans and in the western hemisphere, by far the greatest part of South America, New Spain, and the West-Indea islands, come within the limits. of 30 degrees north and south of the equator. These amazingly large tracts of land imbibe the heat, by which the surrounding air, is rarefied, and thus the wind becomes. constant, or blows in one direction.

You will also remember, that neither the

sea nor the atmosphere are so perfectly transparent as to transmit all the rays of the solar light; many are stopped in their passage, by which both the sea and air are warmed to a considerable degree. These constant or general winds are usually called trade-winds.

Emma. In what part of the globe do the periodical winds prevail?

Father. They prevail in several parts of the eastern and southern oceans, and evi dently depend on the sun; for when the ap parent motion of that body is north of the equator, that is, from the end of March to the same period in September, the wind sets in from the south-west; and the remainder of the year, while the sun is south of the equator, the wind blows from the northeast. These are called the monsoons, or shifting trade-winds, and are of considerable importance to those who make voyages to the East-Indies.

Charles. Do these changes take place suddenly?

Father. No: some days before and after the change there are calms, variable winds, and frequently the most violent

storms.

On the greater part of the coasts situated between the tropics, the wind blows towards the shore in the day-time, and towards the sea by night. These winds are called sea and land breezes; they are affected by mountains, the course of rivers, tides, &c.

Emma. Is it the heat of the sun by day that rarefies the air over the land, and thus causes the wind?

Father. It is: the following easy experiment will illustrate the subject.

In the middle of a large dish of cold water put a water-plate filled with hot water; the former represents the ocean, the latter the land rarefying the air over it. Hold a lighted candle over the cold water, and blow it out; the smoke, you see, moves towards the plate. Reverse the experiment by filling the outer vessel with warm water, and the

move, you will not be surprised at the effects which it is capable of producing. Charles. Is there any method of ascer taining the velocity of the wind?

Father. Yes; several machines have been invented for the purpose. But Dr. Derham, by means of the flight of small downy feathers, contrived to measure the velocity of the great storm which happened in the year 1705, and he found that the wind moved 33 feet in half a second, that is, at the rate of 45 miles per hour: and it has been proved that the force of such a wind is equal to the perpendicular force of 10 pounds avoirdupois weight on every square foot Now if you consider the surface which a large tree, with all its branches and leaves presents to the wind, you will not be surpri sed, that, in great storms, some of them should be torn up by the root.

Emma. Is the velocity of 45 miles an hour supposed to be the greatest velocity of the wind?