the construction of the Nautical Almanac. In this the angular distance of the Moon from the Sun and certain fixed stars is inserted for every third hour in the day, calculated for the meridian of Greenwich. "If, therefore, under any meridian, a lunar distance be observed, the difference between the time of observation and the time in the Almanac, when the same distance was to take place at Greenwich, will show the longitude." The stars selected for the Almanac are nine, viz. the Alpha, or first star of Aries, Aldebaran of Taurus, Pollux of Gemini, Regulus of Leo, Spica of Virgo, Antares of Scorpio, Altair of Aquila, Fomalhaut of Piscis Australis, and Markab of Pegasus. The Nautical Almanac is annually published in England by the commissioners of longitude.

For practice in finding longitude, with the necessary tables, the student is referred to Dr. Bowditch's useful work, the "Practical Navigator."

Except a small variation on account of the spheroidical figure of the Earth, degrees of latitude remain the same, or of equal length, on every part of the globe. But those of longitude decrease from the equator to the poles, where they become extinct. The number of degrees in a circle of longitude is the same in all latitudes; but the number of miles in a degree continually lessens each way from the equator. The student versed in trigonometry may be informed, that the proportion is as radius is to the cosine of any given latitude, so is the number of miles in a degree of longitude at the equator, to the number of miles in a degree of longitude at such latitude.

The table following may be useful, not only as giving the number of miles in a degree of longitude at any distance from the equator; but for comparing geographical with statute miles, 60 geographical and 69 statute miles being the length of a degree of longitude at the equator.

A TABLE OF GEOGRAPHICAL AND STATUTE MILES IN A DEGREE OF LONGITUDE AT EACH DEGREE OF LATITUDE FROM THE EQUATOR.

Deg. Geog. Stat. Deg. Geog. Stat. Deg. Geog. Stat.
lat. miles. miles. lat. miles. miles. lat. miles. miles.
1 59.99 69.49 31 51.43 59.57 61 29.09 33.69
2 59.96 69.46 32 50.88 58.94 62 28.17 32.63
3 59.92 69.40 33 50.32 58.29 63 27.24 31.55
4 59.85 69.33 34 49.74 57.62 64 26.30 30.47
5 59.77 69.23 35 49.15 56.93 65 25.36 29.37
6 59.67 69.12 36 48.54 56.23|| 66 24.40 28.27
7 59.55 68.98 37 47.92 55.51 67 23.44 27.16
8 59.42 68.82 38 47.28 54.77 68 22.48 26.04
9 59.26 68.64|| 39 46.63 54.01 69 21.50 24.91
10 59.09 68.44 40 45.96 53.24 70 20.52 | 23.77
11 58.90 68.22 41 45.28 52.45 71 19.53 22.63
12 58.69 67.98 42 44.59 51.65 72 18.54 21.48
13 58.46 67.72 43 43.88 50.83 73 17.54 20.32
14 58.22 67.43 44 43.16 49.99 74 16.54 19.16
15 57.96 67.13|| 45 42.43 49.14 75 15.53 17.99
16 57.68 66.81 46 41.68 48.28 76 14.52 16.81
17 57.38 66.46 47 40.92 47.40 77 13.50 15.63
18 57.06 66.10 48 40.15 46.50 78 12.47 14.45
19 56.73 65.71 49 39.36 45.60 79 11.45 13.26
20 56.38 65.31 50 38.57 44.67 80 10.42 12.07
21 56.01 64.88 51 37.76 43.74 81 9.39 10.87
22 55.63 64.44 52 36.94 42.79 82 8.35 9.67
23 55.23 63.98 53 36.11 41.83 83
24 54.81 63.49 54 35.27 40.85 84
25 54.38 62.99 55 34.41 39.86 85
26 53.93 62.47 56 33.55 38.86 | 86
27 53.46 61.92 57 32.68 37.85 87
28 52.98 61.36 58 31.80 36.83 88
29 52.48 60.79 59 30.90 35.8089
30 51.96 60.19 60 30.00 34.75 || 90

7.31

8.47

6.27

7.26

5.23 6.06

4.85

4.19
3.14 3.64
2.09 2.43
1.05 1.21
0.00 0.00

Where is the centre of the meridian and other great circles of the Earth? Are the great circles of the globe considered as celestial circles? What may be used, in regard to these circles, in determining latitude and longitude? What two things being known, may the latitude of a place be determined? How may the zenith distance of a heavenly body be obtained? What corrections are required in finding the altitude of the Sun or Moon? How near to the pole star does the north pole of the Earth point?

Is it drawing nearer, or receding? What is meant by semi-diame ter in these calculations? What is meant by depression of the horizon? Does it increase, or not, with the height of the eye? From what is longitude reckoned? What did Philip III. of Spain offer for the discovery of longitude? What other governments offered rewards for this discovery? When was the royal observatory at Greenwich built? What instructions were given to Mr. Flamstead, the first astronomer royal, and his successors, respecting longitude? What did the British parliament offer, in 1714, for the discovery of longitude? What premium did Mr. John Harrison obtain for his time-keeper? Has the government of the United States paid any attention to the subject of longitude? Who, in 1809, presented a memorial to congress on this subject? Where, according to the committee of congress, did the ancient Greeks fix their first meridian? Where did the Arabians establish theirs? In more modern times, what have most European nations done? What would have been fortunate for the science of geography and navigation? Is it likely that all nations will agree upon the same first meridian? Are there any reasons why the United States should establish a first meridian for themselves? In the maps and charts of the United States, is longitude always reckoned from the same meridian? Through what important place ought the first meridian of the United States to pass? What was the opinion of Mr. Monroe on this subject? Where did General Washington intend the first meridian of the United States should pass? How many degrees of longitude east or west make an hour's difference in relative time? Is it earlier in the day, or later, to the east of us? How is it to the west? How can difference of longitude be ascertained by a good time-keeper? Have any timekeepers been found so accurate as to be entitled to perfect dependence? How can the eclipses of Jupiter's satellites be used for determining longitude? Can these eclipses be conveniently used at sea? Are lunar observations for finding longitude perfectly modern? To whom are we indebted for the present state and practical improvement of these observations? Who first proposed and superintended the construction of the Nautical Almanac ? In this, from what is the angular distance of the Moon taken ? How are lunar observations used in determining longitude? What stars are used for these observations? Do degrees of latitude differ in different places? How do degrees of longitude differ? How is the table of longitude useful?

CHAPTER XVI.

Meteors.

In some astronomical works are to be found accounts of lightning, thunder, clouds, aurora borealis, and even of wind, rain, snow, and hail. These, though highly important, and deserving the attention of the chemist and the student in general philosophy, seem not connected with astronomy, nor deserving a place in a work intended to be exclusively astronomical.

But aerolithes, or falling stones, seem worthy of some notice, even in a compendium of astronomy. "It must be reckoned," says Rees's Cyclopædia," among the wonders of the age in which we live, that considerable portions of these heavenly bodies are now known to have descended to the Earth. So wonderful and unexpected an event was at first received with incredulity and ridicule; but we may now venture to consider the fact as well established as any other hypothesis of natural philosophy, which does not actually admit of mathematical demonstration."

He

One of the earliest accounts we have of these phenomena is given by Livy, in his History of Rome. tells us that, in the time of Tullus Hostilius, the successor of Numa, and third king of Rome, it was announced to the king and to the fathers, that it rained with stones on mount Albanus; that these stones fell from heaven not otherwise than when the winds drive the hail thick to the Earth.

Pliny mentions, that a large stone fell in Thrace, in the second year of the 78th Olympiad.

Three large stones are said to have fallen in Thrace, in the year before Christ 452.

It would be useless to dwell on the numerous ac

counts of these phenomena handed down to us from great antiquity. But it may be proper to give a few instances of the falling of these stones in modern times, received on the authority of different authors.

A shower of falling stones, 1200, one of 120 lbs., is related to have happened near Padua, in Italy, in 1510. April 5, 1804, a stone of this kind fell near Glasgow, in Scotland. Several gentlemen of the university well ascertained the particulars of this phenomenon.

But New England affords one of the best authenticated accounts of these wonderful stones. Professors Silliman and Kingsley visited and carefully examined every spot where it was ascertained these stones had fallen. The principal fall was within the bounds of Weston, in Connecticut; though the most northerly was in Huntingdon, on the borders of Weston. Something of the original account deserves to be extracted. "The meteor which has so recently excited alarm in many, and astonishment in all, first made its appearance in Weston, about a quarter or half past six o'clock, on Monday the 14th of December, 1807. The morning was somewhat cloudy, mingled with spots of clear, a space of 150 along the northern horizon perfectly clear; there was little or no light, except from the Moon, just setting.

In

"Judge Wheeler was passing through the enclosure adjoining his house, with his face towards the north, and his eyes on the ground, when a sudden flash across the northern sky made him look up; he immediately discovered a globe of fire, passing behind the first cloud, which was very dark, and obscured the meteor. this situation, its appearance was distinct, like the Sun seen through a mist. Its progress was not so rapid as that of common meteors and shooting stars. When it passed the clear sky, it flashed with a vivid light, not so intense as lightning in a thunder-storm, but like what is called heat lightning. Its surface was apparently conWhen not too much obscured by clouds, a conical train of paler light attended it waving, and in length

vex.