ocentric, is the only one of any use for this purpose, we shall here insert, and explain, so much of that part of Mr. White's ephemeris, as will enable the pupil to rectify his planetarium.

Days.

Day Length Helioc. Helioc. Helioc. Helioc. Helioc. Helioc. increas of Day. long. long. long. long. long. long.

In the foregoing table, for May, 1790, you have the heliocentric places calculated to every six days of the month, which is sufficiently accurate for general purposes. Thus, on the 19th, you have Saturn in 28° 11' of Pisces, Jupiter 3° 37′ of Virgo, Mars in 5° 20′ of Libra, the Earth 28° 36′ of Virgo, Venus 7° 7′ of Capricorn, and Mercury 4° 15' of Virgo; to which places on the ecliptic of the planetarium the several planets are to be set, and they will then exhibit their real situations, both with respect to the sun and the earth, for that day.

To use the Instrument as a tellurian, plate 12, fig. 1. The sun, the earth, and the moon, are bodies which, from our connection with them, are so in

teresting to us, that it is necessary to enter into a minute detail of their respective phenomena. To render this instrument a tellurian, all the planets are first to be taken off, the piece of wheel-work, ABC, is to be placed on in their stead, in such a manner that the wheel c may fall into the teeth that are cut upon the edge of the ecliptic. The milled nut, D, is then to be firmly screwed on, to keep the wheel work firmly in its place. It is best to place this wheel-work in such a manner, that the index E may point to the 21 of June, and then to move the support of the globe so that the north pole may be turned towards the sun, or the 1 of Cancer.

The instrument will then shew, in an accurate and clear manner, all the phenomena arising from the annual and diurnal motion of the earth; as the globe is of three inches diameter, all the continents, seas, kingdoms, &c. may be distinctly seen; the equator, the ecliptic, tropics, and other circles, are very visible; so that the problems relative to peculiar places may be satisfactorily solved. The axis of the earth is inclined to the ecliptic in an angle of 66 degrees, and preserves its parallelism during the whole of its revolution. About the globe there is a circle, HI, to represent the terminator, or boundary between light and darkness, dividing the enlightened from the dark hemisphere. At NO is an hour-circle, to determine the time of sun-rising or setting, lengths of days and nights, &c.

The brass index G represents a central solar ray; it serves to shew when it is noon, or when the sun

is upon the meridian at any given place: it also shews what sign and degree of the ecliptic on the globe the sun describes on any day, and the parallel it describes.

The plane of the terminator, HI, passes through the centre of the earth, and is perpendicular to the central solar ray. The index E points out the sun's place in the ecliptic circle for any given day in the year.

To explain the changes of seasons by the tellurian.

Before I shew how the seasons are explained by the instrument, it is necessary to assume two propositions: 1. That a globular luminous body, sending out parallel rays of light, will only enlighten one half of another globe, and that of course will be the hemisphere turned towards the luminous body. 2. That the earth moves round the sun in such a manner, that in all parts of its orbit its axis is parallel to itself, and has a certain inclination to the plane of the orbit. These being understood, the first thing to be done is to rectify the tellurian; or, in other words, to put the globe into a position similar to that of the earth, for any given day. Thus, to rectify the tellurian for the 21st of June, turn the handle till the annual index comes to the given day; then move the globe by the arm KL, so that the north pole may be turned towards the sun; and adjust the terminator, so that it may just touch the edge of the arctic circle. The globe

is then in the situation of the earth for the longest day in our northern hemisphere, the annual index pointing to the first point of Cancer, and the 21st of June; bring the meridian of London to coincide with the central solar ray, and move the hourcircle, NO, till the index points to XII; we then have the situation of London with respect to the longest day.

Now, on gently turning the handle of the machine, the point representing London will, by the rotation of the earth, be carried away towards the east, while the sun seems to move westward; and when London has arrived at the eastern part of the terminator, the index will point on the hour circle the time of sun-setting for that day; continue to turn on, and London will move in the shaded part of the earth, on the other side of the terminator; when the index is again at XII, it is midnight at London: by moving on, London will emerge from the western side of the terminator, and the index will point out the time of sun-rising, the sun at that instant appearing to rise above the horizon in the east to an inhabitant of London.

It will be evident by the instrument, while in this position, that the central solar ray, during the whole revolution of the earth on its axis, only points to the tropic of Cancer, and that the sun is vertical to no other parts of the earth, but those which are under this tropic.

By observing how the terminator cuts the several

parallels of the globe, we shall find that all those be. tween the northern and southern polar circles, except the equator, are divided unequally into diurnal and nocturnal arches, the former being greatest on the north side of the equator, and the latter on the south side of it.

In this position, the northern polar circle is wholly on that side the terminator which is nearest the sun, and therefore altogether in the enlightened hemisphere, and the inhabitants thereof enjoy a continual day. In the same manner, the inhabitants of the southern polar circle continue in the dark at this time, notwithstanding the diurnal revolution of the earth; it is the annual motion only which can relieve them from this situation of perpetual darkness, and bring to them the blessings of day, and the enjoyments of summer; while in this state, the inhabitants of north latitude are nearest to the central solar ray, and consequently to the sun's perpendicular beams, and of course a greater number of his rays will fall upon any given place, than at any other time; the sun's rays do now also pass through a less quantity of the atmosphere, which, together with the length of the day and the shortness of the night, are the reasons of the increase of heat in summer, together with all its other delightful effects.

While the earth continues to turn round on its own axis once a day, it is continually advancing from west to east, according to the order of the signs, as is seen by the progress of the annual