minute insect. To be fair, let us imagine that the lead ball is as large as the earth and then make the comparison. When this has been done, it will be found that the imaginary ball of lead would have about twice the pulling power of the earth. Hence a ball of lead as large as the earth has a mass double that of the earth itself.

In practical science it is usual to use water as the standard of comparison and to express by the word density or, more accurately, specific gravity, the ratio of the weight of a substance to the weight of an equal volume of water. Bulk for bulk, lead weighs 11.2 times as much as water; this number, therefore, represents the density of lead. And since the earth,

as a whole, has half the mass of a ball of lead of the same size, its density is half that of lead, that is 5.6; in other words, the earth's mass is 5.6 times greater than the mass of a globe of water of the same size. Now a cubic mile of water weighs 410 million tons. There are 260,000 millions of cubic miles in the earth, hence, if our globe consisted of water, its weight in tons would be 410 millions multiplied by 260,000 millions. But since the earth eavier

than water, the result obtained has to be multipled by 5.6 to get the proper value. It is impossible to grasp the significance of the string of figures thus found. Here it is, 6,000,000,000,000,000,000,000 tons. Or, expressed in words, six thousand trillion tons.

The working of this sum must be considered more as a feat of arithmetical gymnastics than anything else. It is not at all necessary to know the earth's weight in tons for astronomical purposes; indeed the very word weight used in such a sense is a misnomer. Weight is simply

due to the earth's mass in comparison with

by other bodies.

SYNOPSIS OF CHAPTER III

The measurement of angles.-The radius of over 360 degrees () in making one revolution contains 60 minutes ('), and each minute 60 sec

Determination of the size of the earth.-The volved are: (1) the measurement of a base triangulation of a tract of country from the determination of the difference of latitude bety at a known distance apart.

The earth's shape is that of a globe slightly the poles. This is proved by (1) the progress the lengths of degrees of latitude from the poles; (2) the increased rate of a pendulum t latitudes

The earth's density. The pull of a lead ball and size is compared with the pull of the ear is calculated to pull with half the force of a lead 1 size, and its mass is, therefore, half what it would of lead. But since density is proportional to n density is half that of lead, that is about 51.

A celestial meridian is a great circle passing through the zenith and nadir of a place and through the celestial poles.

The meridian of a place on the earth is a great circle passing through that place and the terrestrial poles.

The longitude of a place is the angular distance between the meridian of that place and a conventional meridian; latitude is angular distance north or south of the equator.

CHAPTER IV

WANDERING STARS

"AND God said, Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days and years." These are the words of the writer of Genesis, and they reflect the mode of thought common in his day. The motion of the sun across the sky forced itself upon the attention of mankind from time immemorial. In the early morning the orb of heaven appears upon the eastern horizon; he crosses the sky in a majestic sweep, and disappears in the evening. From the time of rising, the ruler of day climbs the sky. It is midday when the highest point is reached, and shadows thrown by sunlight then have their shortest length. The farm-labourer working in the fields observes the fact, and forthwith departs to his midday meal. As the sun slowly sinks towards the west, the shadows increase in length, and finally the twilight shades merge into the darkness of night. These variations are so obvious that they cannot escape notice; hence there is no difficulty in understanding that men ignorant of the cause of the apparent motion of the heavens considered that one of

the chief functions of the sun was to let them know

how the time passed.

The apparent daily motion of the sun is known to all, but there is an apparent yearly motion which is only detected by the observant. In the evening, when the sun has sunk "to shine on other climes," and night has come on, certain groups of stars or constellations will be seen above the western horizon, other groups will be southing, and still other groups will be found in the east. Watch the sky at the same time every night, and in a few days the stars which were seen just above the western horizon at the end of twilight appear on the horizon; those that were due south appear nearer the west, and those that were on the eastern horizon are seen above it. In the course of

seen above the

a few weeks the stars which were western horizon just after sunset will be invisible; those which were on the eastern horizon will be high above it, and other stars will have taken their place. In about six months after the first observation, stars which were on the eastern horizon will appear on the western horizon at the same time, and in a year the sky presents the same appearance as formerly. Leaving for the present the cause of these changes, let us inquire how they were used by ancient peoples to measure time.

Imagine for a moment that we had to look heavenward" for seasons, for days and years," instead of depending upon such mundane contrivances as clocks and calendars. A sun-dial enables us to find the time of day; the passage of the stars across the sky serves the same purpose at night. For the regulation of our seasons let us look to the stars in