meridian in a given time will be to the arc of the equator, passed through in the same time, as 55 to 60.

Mr. Ferguson explains this subject by a very easy problem upon a common globe. If we put small patches of paper, or any mark we please, on every tenth or fifteenth degree both of the equator and the ecliptic, as inscribed on the globe, beginning at Aries v, and turn the globe gently round westward; we shall find all the patches or marks on the ecliptic from Aries to Cancer, come to the brazen meridian sooner than the corresponding marks on the equator; those from Cancer to Libra will come later to the meridian than the marks on the equator; those from Libra to Capricorn sooner; and lastly, those from Capricorn to Aries later again. The marks at the beginning of Aries, Cancer, Libra, and Capricorn, will come to the meridian at the same time with those on the equator; or the same thing may be proved by having two little balls to revolve in equal times, the one in the circle of the ecliptic, and the other in that of the equator.

3dly, The orbit of the earth being an ellipsis, of which the sun occupies one of the foci, the portions of the ecliptic which the sun appears to traverse are not equal to the portions which the earth really passes in its orbit.

These three causes combine at some times to produce the same effect; and at others they

There is also a fourth,

counteract each other. but much minuter modification, occasioned by the precession of the equinoxes. On these four accounts, and on established principles, astronomers compute what are called equation tables, showing when the sun is faster or slower than the clock, or the difference between solar and mean time. These are also given for the noon of every day in the year, in the Nautical Almanac, the Connaissance des Tems, Bode's Jahrbuch, and White's Ephemeris; and for every fifth day, in the Ladies' and Gentleman's Diaries.

Solar time, from these circumstances, can coincide with mean time only at four periods of the year, viz. about the 16th of April, the 15th of June, the 31st of August, and the 25th of December. On all the other days of the year some inequality will be found, as shown by the equation tables.

We have seen that the day consists of twentyfour hours of mean time.-But the term artificial day is used to express the interval in which the sun is above the horizon; and the time during which the sun is below the horizon, is called night. The artificial day, it is well known, is of different duration in different regions of the globe, and at different seasons of the year. To those who live under the equator it is exactly twelve hours. At the pole the artificial day is of six months' duration; and at all those regions

which lie between the equator and the poles, it is subject to considerable variations.

To these parts of the globe the artificial day is only of 12 hours' length when the sun is in one of those points called the equinoxes, where the equator intersects the ecliptic. At other times, it is either longer or shorter, according to circumstances. To those who live between the equator and the north pole, the day becomes longer than 12 hours, in proportion as the sun continues to advance from the equator to the tropic of Cancer, that is, from the time of the vernal equinox; on the contrary, it becomes shorter than that period, in proportion as the sun advances from the equator towards the tropic of Capricorn, which takes place after the autumnal equinox. The contrary happens to the inhabitants of the southern hemisphere. Thus, in all the regions on either side of the equator there are only two days in the course of the year when the day and night are equal: and throughout the earth, except in the frigid zones, the longest and the shortest day together make 24 hours.

Such is the duration of the artificial day to the different inhabitants of the earth, if we regard only the actual presence of the sun above the horizon. But there is one cause, which serves to prolong the day-light, which we have not yet noticed, and that is refraction. After what you have read in the optical lectures, you

can scarcely be at a loss to understand its operation. From this cause the sun appears, both at rising and setting, above the horizon, when he is actually below it. Let us suppose (fig. 4) T to be the earth, t≈ the mass of the atmosphere, S the sun just below the horizon, H h; suppose the ray Se then to be emitted from that luminary, and to arrive at the atmosphere e, which must be of greater density than the medium through which the ray has hitherto been transmitted. The ray is consequently refracted towards the perpendicular pp, and reaches the spectator at t, who then sees the sun in the direction ts; he sees him, therefore, in a situation nearer the zenith than he really is.

But since the atmosphere is not every where of the same density, and since its density increases as it approaches the surface of the earth, the ray Da, for instance, must suffer many successive refractions, and arrive at the spectator t in a kind of curve a b c t; and if the line t d is the tangent to this curve, the observer will see the luminous body D elevated to d. The effect of refraction in our climate is to cause the sun, when he is in the horizon, to appear about 32 or 33 minutes of a degree higher than he really is; whence it follows that he will appear above the horizon when he is actually below it.

I have said that the artificial day comprehends all that space of time during which the sun is above the horizon; but if we apply this

term to the duration of light, it must also include the twilight, and the length of the artificial day will be proportionably extended.

Twilight is that portion of light which the sun diffuses through the atmosphere before his rising and after his setting. The morning twilight, which the ancients distinguished by the name of Aurora, introduces to us the dawn of morning when the sun is about 18 degrees below the horizon; and the evening twilight disappears when the sun has descended about 18 degrees lower than the horizon. Thus the line of twilight may be regarded as a circle parallel to the horizon, and about 18 degrees below it.

The morning twilight continues to augment as the day advances; that of the evening, on the contrary, decreases gradually till it totally disappears. I have said the twilight is the effect of the dispersion of the solar rays in the atmosphere, by which they are refracted and reflected in various directions. To understand this, let T (Pl. II. fig. 5) be the earth, A A A the atmosphere, HH the horizon, C C C a vertical circle, which serves to measure the height of the sun, S the sun below the horizon either before his rising, or after his setting. The solar rays Ss, Ss, S s, Ss, are directed towards the points, BBB B, and they would pursue this direction but for the interposition of the atmosphere, which having more density than the medium through which they were last transmitted, and the rays