If the earth were a uniform surface, every place on the same parallel of latitude would have exactly the same climate.

Geographers have divided the surface of the globe into zones, or belts, separated by imaginary circles parallel to the equator. The middle is called the torrid zone, on each side of which there is a temperate zone, succeeded by a frigid zone.

According to this division, our own country is in a temperate zone.

But this is only one cause of the difference of climate; there are several others.

Other causes are

1. The relative position of land and water.-The waters of the ocean are of very equal temperature, and have therefore a tendency to preserve equal temperature where their influence extends. A hot wind is cooled, and a cold wind warmed, as it passes over the sea. Hence the climate of islands is generally more equable than that of inland countries of the same latitude. The winters are milder, and the summers more temperate.

There is much more water in the southern than in the northern hemisphere, and the result is that the temperature is more equable.

2. The elevation of the ground above the level of the sea.-As we ascend mountains, cold always increases; and so, even in tropical countries, the highest mountains are covered with perpetual snow; and by descending such a mountain, we should pass through all the varieties of temperature, from a polar to a tropical climate.

The great Plain of Quito, though lying under the Equator, enjoys a temperate climate, because it is at an altitude of 9000 feet above the sea-level.

3. The position of mountain ranges, and the direction in which a country slopes, affect the climate. The southern slopes of the Alps are cultivated 1000 feet

higher up than the northern, because they receive the rays of the sun more directly. The Alps contribute to the warm climate of Italy, by sheltering it from the cold northern winds. On the contrary, Erzeroum in Armenia, which is south of Naples, has almost a polar climate, because it is situated amidst the snow-capped ridges of Mount Ararat.

4. The prevailing winds. The west and south winds sweep over the Atlantic Ocean, and are therefore mild in their character, and tend to moderate the cold of the countries in the West of Europe. The hot southerly wind, called the sirocco, so common in countries round the Mediterranean, owes its character to the Great Sahara, over which it passes; and as the Mediterranean is not large enough to cool it, this wind contributes much to make these countries warm.

The winds in the eastern counties of England come from the vast plains of Germany, and have only the waters of our narrow Channel to temper them; consequently in winter and spring, when these plains are damp and chilly, the east winds are cold and bleak. But in the autumnal months the plains of Germany are dry and sandy, and therefore at this time our east winds are likely to be hot and parching.

5. Ocean Currents.-As the earth moves round from west to east, and the ocean can move freely in its bed, there would from this cause be an ocean-current apparently flowing from east to west. But this rotation of the earth produces also another effect: it causes the water to flow from the poles towards the equator. Thus we should have two great movements in the oceanthe one from pole to equator, the other from east to west. Besides, water has less density when heated than cold. So water being warmed at the equator is made lighter, and flows away towards the poles. Thus there is a continual interchange of hot and cold water over the earth. These currents are variously affected by

other currents, by the shores against which they strike, and by other causes.

The most powerful of all currents is that known by the name of the Gulf Stream. This issues from the Gulf of Mexico, where the water becomes heated as in other inland seas of warm latitudes; and so a continual current of warm water flows through the Straits of Bahama, in a north-easterly direction, at the rate of three or four miles an hour. Its speed diminishes as it proceeds farther north, and it is finally lost near the shores of Newfoundland, where it meets the great polar current. The Gulf Stream retains considerable warmth for a great part of its course, and is thought to be one of the causes which render the climate of our western shores more mild than that of the eastern.

Petropaulskoi in Kamtschatka, whose harbour is blocked up with ice from November to May, lies in the same latitude as Wicklow in Ireland.

There are, besides, other causes which affect the climate of a particular place,-among which are the nature of the soil and the neighbourhood of forests.

CHAPTER XLVII.

VENTILATION.-No. 1.

WHAT is Ventilation? Few people if any know exactly what ventilation is, or how to produce it. They know from the first part of the word that it has something to do with the wind, or the air, but what it is they do not clearly understand.

Some people appear to think that ventilation consists in keeping all places as cold as possible, and are quite ignorant of the fact, that in many cases a place cannot be ventilated without the aid of warmth.

Others admit the use of warmth, but think the supply of warm air must be counterbalanced by a large amount

of cold air, and so condemn people to live in a constant draught.

People generally do not seem to be aware that the essential part of ventilation is movement of the air.

Of course, if hot air, or any air, has been removed from a part of a room, it follows that another portion of air will rush in; and although, generally, the removed air is less pure than the new supply, this is not always so; for ventilation is required even in the open air, and in enclosed places where all the air is of equal purity.

No ventilation can be effected without movement of the air; and this movement is always followed by certain consequences necessary to our comfort or our health. We provide for these, then, when we provide for motion of the air.

So important are these consequences, that even if the new supply which rushes in to take the place of that which has been removed is less pure, the movement is still to some extent beneficial. The foundation of ventilation in the air, therefore, is movement.

The air is composed of several gases, which could never exist in contact with each other so as to form an atmosphere, without movement, the necessity for which arises out of their very nature.

Air is as important to human beings as water is to fish. Fish cannot live in stagnant water, nor can human beings exist in a proper condition in stagnant air. It requires the help of the motion in the air to rid itself of heat and moisture through the skin, and-principally by means of the lungs-of certain poisonous and offensive vapours formed in its interior.

As regards heat-the body is warmed, chiefly from within, by the changes which our food undergoes; and as, notwithstanding any excess there may be in the supply of heat, the body remains generally at about the same temperature, it cannot be doubted that it rids itself of the superfluous heat by discharging it into the air.

Air is, however, a bad conductor of heat-that is, it does not carry it away quickly; and when the excess of heat thrown out by the body is very great, the layer of air close to the body cannot remove it rapidly enough, but, in proportion to the rapidity of the movement, the heat will generally be more or less rapidly dispersed.

This may be proved by using a fan in a hot room. The fanning does not cool the air about the body; it merely puts it in motion, and drives the heat a little distance off, by the passage for it the movement makes through the air.

In hot climates-such as India, &c.—immense fans, called "punkahs," are fixed to the ceiling in churches and public buildings. Constantly moved, these keep the body comparatively cool, and it will be seen that this is not done by a supply of fresh air, but merely by the movement of the punkah; for when the use of the punkah is required, the outside air is so hot as to make it necessary to close any aperture which could admit it.

Our sense of comfort, and the state of our spirits, and therefore of our health, depend to a great extent on movement of the air. If we enter a large room which has been shut up for a long time, with a sniff of the air we remark its want of freshness. If we stay in it we feel low-spirited and unhappy; and yawning, and perhaps a headache, follows; but introduce a punkah, or adopt any mode by which the air is set in motion, and at once a sense of relief is experienced.

So, the body requires that the air be kept in motion; but the air itself requires it also. Motion is necessary to keep it of a uniform character, and in a fit state to preserve health.

Every house, every room, every closet in a house, should frequently be exposed to air in motion; a closed house is generally a damp house.

The lower layer of the atmosphere holds more moisture than the higher layers. The air surrounding living