may be answered, that our summer is hotter than the winter; first, on account of the greater height to which the sun rises above our horizon in the summer; secondly, the greater length of the days. The sun is much higher at noon in summer than in winter; and, consequently, as its rays in summer are less oblique than in winter, more of them will fall upon the surface of the earth. In the summer, the days are very long, and the nights very short; therefore, the earth and air are heated by the sun in the day-time, more than they are cooled in the night; and upon this account, the heat will keep increasing in the summer; and for the same reason will decrease in winter, when the nights lengthen.

I should exceed the limits of this Essay, if I were to inquire into the several concurring causes of the temperatures that obtain in various climates; it may be sufficient, therefore, to observe what a remarkable provision is made in the world, and the several parts of it, to keep up a perpetual change in the degrees of heat and cold. These two are an tagonists; or, as Lord Bacon calls them, "the very hands of Nature with which she chiefly worketh ;" the one expanding, the other contracting bodies, so as to maintain an oscillatory motion in all their parts; and so serviceable are these changes in the natural world, that they are promoted every year, every hour, every moment. From the oblique position of the ecliptic, the earth continually presents a different face to the sun, and never receives his rays two days together in the same direction. In the

day and night, the differences are so obvious, that they need not be mentioned; though they are most remarkable in those climates, where the sun at his setting makes the greatest angle with the horizon. Every hour of the day the heat varies with the sun's altitude, is altered by the interposition of clouds, and the action of winds; and there is little room to doubt, but what the various changes that thus take place, concur in producing many of the smaller and greater phenomena of nature.

Be this however as it may, it is certain that the various irregularities and intemperature of the elements, which seem to destroy nature in one season, serve to revive it in another: the immoderate heats of summer, and the excessive cold of winter, prepare the beauties of the spring, and the rich fruits of autumn. These vicissitudes, which seem to superfieial minds the effects of a fortuitous concourse of irregular causes, are regulated according to weight and measure by that SOVEREIGN WISDOM, who weighs the earth as a grain of sand, the sea as a drop of

water.

OF THE SOLAR AND SIDERIAL TIME.

I have already shewn, that the daily motion of the sun from east to west, is not a real but an apparent one; which is owing to the rotation of the earth round its axis. Now if the sun had no other motion but this apparent one, it would seem to go once

round the earth, in the time of one complete rotation, or in 23 hours, 56 minutes; which is the case with any of the fixed stars, and is therefore the length of a siderial day. But the sun is found to take up a longer time to complete its apparent revolution; for if it is in the south of any particular place at twelve o'clock at noon to-day, it will not complete an apparent revolution, so as to return to: the south of that place again, till twelve o'clock at noon on the next day; and, consequently, the time of this apparent revolution is twenty-four hours.

Let us endeavour to render this subject clearer, by defining, in other words, the nature of the solar and siderial day.

The solar day is that space of time which intervenes between the sun's departing from any one me-ridian, and its return to the same circle again; which space is also called a natural day; or it is the time from the noon of one day to the noon of the next.

The siderial day is the space of time which happens between the departure of a star from, and its return to, the same meridian again,

I am now to shew why these days differ in length, or why the time, that the sun takes up to complete one revolution, is longer than the time the earth takes to revolve once upon its axis.

This difference arises from the sun's annual motion. For the sun does not continue always in the same place in the heaven, as the fixed stars do; but it is seen at M, plate 4, fig. 2, one day, near the fixed star R; it will have shifted its place the next

day, and will be near to some other fixed star L. This motion of the sun is from west to east, and one entire revolution is completed in a year. Suppose, therefore, that the sun, when it is at M, near to the fixed star R, appears in the south of any particular place S; and then imagine the earth to turn once round upon its axis from west to east, or in the direction of STVW, so that the place may be returned to the same situation; after this rotation is completed, the star R will be in the south of the place as before; but the sun having, in the meantime, moved eastwards, and being near to the star L, or to the east of R, will not be in the south of the place S, but to the eastward of it: upon this account, the place S must move on a little farther, and must come to T before it will be even with the sun again, or before the sun will appear exactly in the south.

This may be illustrated by an instance. The two hands of a watch are close together, or even with one another at twelve; they both turn round the same way, but the minute-hand turns round in a shorter time than the hour-hand; when the minute-hand has completed one rotation, and is come round to twelve, the hour-hand will be before it, or will be at one; so that the minute-hand must move more than once round, in order to overtake the hour-hand, and be even with it again.

As this subject is of some importance, we shall endeavour to render it more clear, by placing it in a different point of view: the more so, as it may ac custom the young pupil to reason on both hypotheses,

namely, the motion of the sun, and that of the earth.

The diameter of the earth's orbit is but a physical point, in proportion to the distance of the stars; for which reason, and the earth's uniform motion on its axis, any given meridian will revolve from any star to the same star again, in every absolute turn of the earth upon its axis; without the least perceptible difference of time being shewn by a clock which goes exactly true.

If the earth had only a diurnal, without an annual motion, any given meridian would revolve from the sun to the sun again, in the same quantity of time as from any star to the same star again; because the sun would never change his place with respect to the stars. But, as the earth advances almost a degree eastward in its orbit, in the time that it turns eastward round its axis, whatever star passes. over the meridian on any day with the sun, will pass over the same meridian on the next day, when the sun is almost a degree short of it; that is, 3 minutes, 56 seconds sooner. If the year contained only 360 days, the sun's apparent place, so far as his motion is equable, would change a degree every day; and then the siderial days would be just four minutes shorter than the solar.

Let ABCDEFGH, plate 4, fig. 3, be the earth's orbit, in which it goes round the sun every year, according to the order of the letters; that is, from west to east; and turns round its axis the same way, from the sun to the sun again, in every twenty-four