coming in an oblique direction, they are refracted towards the perpendicular, and inclined to t t t t. As the sun descends, fewer rays reach the surface of the atmosphere, so as to take this direction. The twilight, therefore, gradually diminishes, and at last totally disappears in the manner already stated.

The duration of twilight is not the same at all places, nor even at the same place in all seasons; since, at certain places, and at certain times, the sun rises and descends perpendicularly from the horizon, while at others his motion both at rising and setting is more oblique, and the most so when his declination is the greatest, at which period it is plain that he must occupy more time in rising or descending a space which shall measure 18 degrees in the vertical direction.

The sun appears to traverse 15 degrees of the equator, or of some circle parallel to it, every hour; twilight therefore must endure for one hour and twelve minutes in those places where the sun rises or sets perpendicularly to the horizon, which is at the time of the equinoxes to those who live under the equator, or who have a right sphere. The length of twilight increases in proportion as the sun is further from the equator, or has more declination.

We must therefore conclude, that to those who live between the equator and one of the poles, or who have an oblique sphere, the length

of the twilight must be greater in proportion as the pole is more elevated above the horizon, or in proportion as the place has a higher latitude. Thus, if the latitude is such that the sun at midnight on the 21st of June cannot have descended eighteen degrees below the horizon, as is the case in our climate, the evening twilight will not be finished before that of the morning commences; so that at this period of the year we have, properly speaking, in clear weather, no night at all. This is indicated in our best Almanacs, in which the length of the morning and evening twilight is given, for every fifth day in the year.

It follows also, that if there were any inhabitants directly under the poles, who would, you know, have a parallel sphere, the twilight would be perceived by them two months before the sun appeared above their horizon, and would also endure two months after the disappearance of the sun. In a parallel sphere, therefore, there would not be more than two months of actually dark night; besides that, during these two months, the moon would be twice visible above the horizon for fourteen days at a time.

The astronomical day commences at noon of true time, that is, at the instant when the sun is in the meridian. In astronomy, the twenty-four hours of the day are counted from noon to the following noon again; so that instead of calling the hour after midnight one o'clock in the morn

ng, as it is shown by the clock, it is termed the thirteenth hour of the day, and so on to the end of the twenty-four hours.

With respect to the civil day, its commencement has not been established by all nations at the same period. The Babylonians began to count the hours of the day from the rising of the sun. The Jews and Greeks, on the contrary, counted from his setting; and this, I believe, is the practice in Italy at present. All the other countries of Europe begin their day as we do, immediately after midnight.

In the Latin, the seven days of the week are denominated after the sun, moon, and the primary planets. With the exception of Saturday and Sunday, the French follow the same plan. In four of our days, Tuesday, Wednesday, Thursday, and Friday, we have adopted the names of four of the Saxon divinities, who are said to correspond with Mars, Mercury, Jupiter, and Venus; Saturday, Sunday, Monday, are the same as the Roman names.

Four weeks two days and about 7th constitute a mean solar month, that is, the time in which the sun appears to traverse one sign, or twelfth part of the zodiac. The actual lengths, however, vary, by reason of the earth's variable motion in different parts of its elliptical orbit.

Twelve months constitute a year, or that period in which the earth completes its orbit round the sun, or in which the sun appears to have

passed through the twelve signs of the zodiac. The year, therefore, consists of 365 days and about. It was at first estimated at only 365 days. But as it was found that, while the earth makes a complete revolution in its orbit, it makes with respect to the sun 365 turns and nearly upon its own axis, which makes the year to include 365 days and six hours, and also that the equinoxes receded about the value of a day in every four years; to remedy this inconvenience, was agreed to employ the six supernumerary or intercalary hours in giving an additional day to every fourth year, which on that account is called leap year, and contains 366 days.

The year, however, consists only of 365 days, 5 hours, 48 minutes, and 45 seconds. In adding, therefore, to every fourth year a whole day, we give 11 minutes and 14 seconds too much. This quantity, though small in itself, must in the course of many years amount to a considerable sum. In the pontificate of Gregory XIII, the equinoxes had advanced ten days, so that the vernal equinox, instead of the 20th, fell on the 30th of March. This irregularity caused much inconvenience with respect to the festivals of the church. The pope, therefore, with the advice of the ablest astronomers, reformed the calendar. Having calculated that the surplus time, viz. eleven minutes and fourteen seconds, which had been added to the leap years, must amount to a whole day in 133 years, and that, consequently,

from the time of the Nicene council, A. D. 325, the equinox had fallen back eleven days, they struck off at once eleven days from the current year, so that the vernal equinox then fell on the 21st of March; and it was further agreed to omit three of the leap years in the course of 400 years. This arrangement has already been put in practice; since the years 1700 and 1800, which would have been leap years in the usual course, were not observed as such. Even this plan is, however, not quite perfect, for, since the eleven minutes and fourteen seconds compose an entire day at the end of 128 years, instead of 133, this will still cause an error of one day in 3200 years, so that at the end of the year 4800, it will be again necessary to retrench another leap year.

The alteration took place in Britain, in 1752, when what is denominated the New Style was introduced, and 11 days were at once dropped. Another was dropped in the year 1800; so that now, old Lady Day, as it is termed, occurs twelve days after new Lady Day; and the same, of course, happens with regard to old and new Michaelmas day, old and new Martinmas day, &c. The Russians still continue to employ the old style; all other European nations have long ago adopted the new.