from each other by definite climatic differences, as far as climate depends upon astronomical position. These belts or climates are characterized by a certain extension in the length of the longest day. Between the equator and the polar circles, they are defined by the longest day in the parallel of latitude nearest the pole, exceeding by half an hour the longest day at the parallel nearest the equator; and between the polar circles and the poles, where great differences rapidly succeed each other, the excess of a month is adopted as the standard of distinction. There are twentyfour climates thus discriminated between the equator and each polar circle; six from thence to each pole; or thirty in each hemisphere : TABLE OF SUCCESSIVE CLIMATES DIFFERING BY HALF-HOURS FROM THE EQUATOR TO THE POLAR CIRCLES. TABLE OF SUCCESSIVE CLIMATES DIFFERING BY MONTHS FROM THE POLAR CIRCLES TO THE POLES. The circumstances by which the astronomical causes of differential climates are variously modified are hereafter noticed. 70. Latitude. The latitude of a place on the earth's surface is its direct distance from the equator, north or south, according to the hemisphere in which it is situated. It is reckoned in degrees, minutes, and seconds, on a meridian circle, and cannot exceed 90°, the polar distance. Thus the Observatory at Greenwich is situated in 51° 28′ 40′′ north latitude: the Observatory at Paramatta in 33° 48′ 49′′ south. The lengths of degrees of latitude would all be equal to each other, but for the slight polar compression, as stated in a preceding paragraph (52). As the latitude of a place is always equal to the elevation of the pole, it is readily found in the northern hemisphere, by taking the altitude of the polar star with the quadrant, and making allowance for its aberration of 1° 32′ from the true polar point. 71. Longitude.-The longitude of a place is its distance east or west from some selected meridian. The ancients, supposing the earth to be longest from east to west, applied the term longitude to measures in that direction. As nature supplies no particular spot from which longitude may set out in preference to another, nations have varied much in their choice of this station. Ptolemy fixed his first meridian at the Fortunate or Canary Islands; the Dutch at the Peak of Teneriffe; and Mercator selected the island of Corvo, one of the Azores, because there the magnetic needle, in his time, showed no variation. The French now reckon from the meridian of the Observatory at Paris; the Spaniards from that of Cadiz; the English from that of the Observatory at Greenwich. The meridian of Greenwich being our starting-point, the distance of places east or west is reckoned from it in degrees, minutes, and seconds, on the equator, or on a parallel of latitude. Thus the longitude of Paris is 2° 20′ 22′′ east of Greenwich; and that of Washington 77° 2' 0" west. Longitude being reckoned each way, it follows, that as the circumference of the earth is 360°, no place can have more than half that distance, or 180° of longitude. TABLE OF FIRST OR PRIME MERIDIANS OF LONGITUDE ... about 17° 0' 0" W. 700w. about Western extremity of Africa as then known, taken by Abulfeda, ... ... 27 10 0 w. 16 30 0 W. 17 30 0 W. 72. The value of degrees of longitude varies according to the latitude, their lengths diminishing as we recede from the equator. An artificial globe renders this sensible to the eye towards the poles, the meridians gradually approaching each other, and converging in the polar points, where there is no longer any difference of longitude. TABLE SHOWING THE LENGTH OF ONE DEGREE OF LONGITUDE FOR EVERY DEGREE OF LATITUDE IN GEOGRAPHICAL AND ENGLISH MILES. It is clear, therefore, that to reduce the distance in longitude between two places agreeing in latitude to common measures, we must refer to the number of geographical or English miles which belong to a degree of longitude in that latitude; or, what will amount to the same thing, we must transfer the arc between them to the equator, count the degrees it will describe on it, and multiply them by the number of miles which belong to a degree on a great circle. It would, however, be a great mistake to consider the difference of longitude between places having the same latitude, and estimated in this way, as always the measure of their shortest distance from each other. Petersburg and Fort Alexander, in Russian America, furnish a striking illustration. They are both nearly on the same parallel, 60°. Their difference of longitude is about 180°, equivalent, at that parallel, to 6,300 miles; but they are only separated by 60° of latitude counted on a meridian, which is nearly common to both, equal to 4,200 miles. It is true, that to pass from one to the other in the line of the meridian the polar ice must be traversed. 73. The earth accomplishing a diurnal rotation in 24 hours, the sun appears to perform a circuit round it in that time, moving over 1° of longitude in exactly four minutes. Consequently, the mean time at two places apart from each other by 1°, 15°, 30°, etc., of longitude, will differ four minutes, one hour, two hours, etc., the time being so much earlier at the eastern station, and later at the western. Thus the extreme longitudinal limits of the British isles are 1° 45′ east of Greenwich, near Lowestoft, on the coast of Suffolk; and 10° 28′ west, at Cape Sybil, on the west coast of Munster; the difference amounting to 12° 13'; so that the difference of time is equal to nearly 49 minutes. Hence, when it is noon at Cape Sybil, it is 49 minutes past noon at Lowestoft. 74. The determination of longitude is not a simple problem like that of latitude. The equator from which latitude is reckoned can be traced in the heavens, for its pole is that immovable point of the sky, which, though not absolutely occupied by a star, is nearly so, and can be detected by observation of the apparent motion of the stars round it. But there is no appearance in the heavens which defines the meridian of Greenwich, or any other line of terrestrial longitude; and hence the difference of the two problems. To enable the mariner to find his longitude E at sea, and thus shape aright the course of his vessel, is one of the principal objects of the "Nautical Almanac," published yearly by authority, under the care of the Astronomer Royal, but always three years in advance, for the convenience of navigators to far-distant stations. The whole question of the difference of longitude between two places resolves itself into knowing precisely their difference of time at the same instant. This ascertained, the differing time is readily converted into degrees; and earlier or later time shows the eastward or westward position of each place. Certain celestial phenomena are therefore calculated beforehand, and set down in the tables of the almanac at the time they would be seen at Greenwich; and a faraway observer, by comparing with it the time of their actual occurrence at his own station, ascertains the difference, and finds his longitude. Thus the distances of the centre of the moon from certain remarkable fixed stars convenient to her path, are shown to the nearest second of a degree, for every three hours, for three years in advance, and set down according to Greenwich time; and mariners deduce their longitude from the differences of their own times of observing these lunar distances. But both by land and sea, there are intervals when no astronomical observation of any kind can be taken, owing to the state of the atmosphere; and upon the ocean it is often impossible to take an observation with precision, because of the unsteadiness of the station on board a vessel, when all other circumstances are propitious. Hence further aid is obtained by chronometers carrying out Greenwich time, or that of any other known position, by which, indeed, the problem of the longitude might always be solved, provided that these instruments could be absolutely secured from error as time-keepers, and the local time at the station of inspection is carefully ascertained. Another method by which the difference of longitude between two positions within a limited distance may be accurately known, is by signals, the discharge of a rocket, or an explosion of gunpowder. A rocket may be readily seen during a serene night over sea at the distance of fifty or sixty miles; and the flash of a small portion of powder on a highly-elevated station quite as far. Two observers, with the signal at an intermediate point, their clocks regulated according to local |