atmosphere, tend to accumulate to some extent in the lower part of the room. The principle mentioned above is, undoubtedly, correct in rooms where the foulness is caused by gas burning, for the cause of the foulness is usually high up in the room. In greatly crowded places, also, the principle is applicable, for if the heat and foulness be great, the movement produced by the occupants is great also; but for ordinary bedrooms at night, without gas or fire, and with few persons sleeping in a room, it is not applicable.

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In such conditions it is true that heat is produced, and that heated air ascends; but as the increase in the quantity of heat is not great, and as the ascending current of the heated air meets air which has not been heated, it gives out a portion or the whole of its warmth, and its ascent is hindered, if not altogether stopped.

The higher the room is, and the less the degree of heat generated in it, the weaker will be the rising current of air, and the smaller chance will there be of its escaping by ventilators in the ceiling; whilst, on the contrary, the greater the supply of heat, and the lower the room, the stronger will be the ascending current, and the greater the quantity of air which will

escape.

Hence, the success of this system is not the same under all circumstances, nor is it in any case perfectly efficient in removing the heat, unless it is worked in connection with openings in the lower part of the room→→→ in that part of the room where the air is rendered foul.

In practice, it is best to consider that air will both enter and leave by openings which communicate with the outer air, no matter where the openings may be placed; and with most of the doors and windows of our houses, if air be removed, the deficiency will be immediately supplied.

But, in order that heated air may be quickly removed, it is essential that there should be openings at the upper part of the room; and in order to prevent the air in the lower part from becoming stagnant, openings in the lower part of the room also are essential.

The movement of the air, although so essential to health, must not be very perceptible, lest it should cause discomfort and disease. Hence the inlets and outlets must not be very near to the inmates, and the current must be divided; that is, the opening for inlet and outlet must not be so placed, with regard to each other, that the current would rise in an uninterrupted line. The air introduced must circulate and mix with the air in the room before it finds its way to the outlet.-Adapted from Journal of the Society of Arts, Paper 849.

CHAPTER XLIX.

THE TERRESTRIAL GLOBE.

WHEN We speak of a Terrestrial Globe, we usually mean a globe made of plaster of Paris, over which is a covering of paper, upon which are drawn the various countries, lands, and seas of the real earth, in their proper relative positions and proportions.

Suppose you wished to copy a pattern from one sheet of paper to another, the most convenient way would be to draw upon each sheet a number of lines, equally distant from each other, across the paper both ways. If the two sheets were lined exactly alike, you could easily find out where to put every point in the pattern, by measuring its distance from some one of the lines.

If it is wished to make the same pattern only half the size of the original, the same number of lines must be drawn on the blank paper as on the sheet to be copied; but the distance between the lines must be

only half as great, and by making proper measurements from the lines, the right places for each point in the pattern could be found.

This is the way people copy maps. There are certain lines drawn across the paper, and the distances between these represent so many miles; and if a small copy of a large map is being made, all that is necessary is to get the first lines right, and the rest will be easy. In every map there should be a line, along which is marked what length represents a mile, five miles, ten miles, and the like. This line is called the scale of the map.*

Now, if it is desired to map out a globe, some way must be discovered of dividing it by lines, from which the measurements can be made.

The Equator furnishes one line from which to start. Circles parallel to the Equator may be drawn at equal distances from each other, and from the Equator to either Pole; and if it is known how large the earth is, it is easy to calculate how many miles the distances between any two of these circles will represent..

Thus, if the earth were 24,000 miles round, the distance from Pole to Equator would be 6000 miles. If five such circles as have been described are drawn between Equator and Pole, the distance between each circle measured upon the globe will be 1000 miles; and if it were conceded that Cape Verde is 1000 miles north of the Equator, it must be placed somewhere upon the parallel circle next above the Equator.

Any number of great circlest may be drawn passing through the Poles. Let one pass through the spot

*The teacher is particularly advised to exercise his pupils in simple illustrations of the use of scales in map-drawing before he proceeds further with the Lesson. This may be easily done upon a black board from any large map hanging in the schoolroom.

† A great circle is one which divides the globe into equal parts or halves. The equator, ecliptic, and the meridians of longitude are great circles.

where Greenwich is to be marked; this is the meridian of Greenwich. When it is known how far Greenwich is north of the Equator, the proper distance must be measured along the circumference of this circle, from the point at which it cuts the Equator. If, also, the distance of the meridian of Cape Verde from that of Greenwich is known, that distance can be measured along the Equator, and the meridian passing through that point will pass through Cape Verde; and so Cape Verde must be the point where this meridian cuts the parallel circle on which Cape Verde was to be. In this way all other places can be determined, and a globe can be mapped out.

The Equator is about 24,000 miles round; so that, if the artificial globe be one foot in diameter, the circumference of a great circle will be about three feet, and a quarter of this circumference (9 inches) will represont 6000 miles. If the globe be 2 feet in diameter, the circumference of a great circle will be about 6 feet, and a quarter of that circumference (18 inches) will represent 6000 miles.

It remains, therefore, only to decide upon the manner of subdividing the Equator, and whatever size the globe may be, we can measure upon it the distances required. We divide the Equator into 360 parts, called degrees. Each degree is about 693 common English miles. And in the same way the circumference of every great circle is divided into 360 parts.

By the latitude of a place is understood its distance north or south of the Equator.

If a meridian be drawn through the place, and the circumference is measured from the point at which it cuts the Equator to the spot upon which the place is, the latitude of that place will be shown.

London is in 51 degrees north latitude-i. e., if a meridian be drawn through London, and be measured upwards from the Equator to London, the distance

would contain 51 of those parts, of which there are 360 in the whole circumference.

Botany Bay is 34 degrees south latitude-i. e, if a meridian be drawn through Botany Bay, and be measured downwards from the Equator to Botany Bay, the distance will contain 34 such parts of the circumference.

The Poles are the points most distant from the Equator, and no place can have a greater latitude, north or south, than 90 degrees.

Longitude means the distance, measured upon the Equator, from some fixed meridian. In England, longitude is reckoned from the meridian of Greenwich; to the right of this circle the longitude is called east, and to the left west.

Thus Achil, in Ireland, is on the 10th degree of west longitude; Lyons, in France, is on the 5th degree of east longitude-i.e., the Lyons meridian cuts the Equator at a distance of 5 degrees east of the Greenwich meridian, and the Achil meridian cuts the Equator at 10 degrees west of the Greenwich meridian.

As longitude is measured along the Equator to right and left, the east longitude and west longitude meet just halfway round, or exactly opposite the meridian from which the longitude was reckoned; and no place can have a greater longitude, east or west, than 180 degrees.

The meridians drawn upon artificial globes are called circles of longitude, and the parallel circles are called circles of latitude.

An almost unlimited number of meridians might be drawn, but it is convenient to draw twelve such circles, all of which pass through the poles, and divide the Equator into 24 equal parts of 15 degrees each.

Since the earth turns round its axis from west to east once in twenty-four hours, it turns through 15 degrees in one hour. So, when it is noon at Greenwich, it is