poles or the equator) have this position of the sphere, and the days and nights are of unequal lengths, the parallels of latitude being divided into unequal parts by the rational horizon.

69. CLIMATE is a part of the surface of the earth contained between two small circles parallel to the equator, and of such a breadth, that the longest day in the parallel nearest the pole, exceeds the longest day in the parallel of latitude next the equator, by half an hour, in the torrid and temperate zones, or by a month in the frigid zones; so that there are 24 climates between the equator and each polar circle, and six climates between each polar circle and its pole.

From the above definition it appears, that all places situated on the same parallel of latitude are in the same climate; but we must not infer from thence, that they have the same atmospherical temperature; large tracts of uncultivated lands, sandy deserts, elevated situations, woods morasses, lakes, &c. have a considerable effect on the atmosphere. For instance in Canada, in about the latitude of Paris and the south of England, the cold is so excessive, that the greatest rivers are frozen over from December to April, and the snow commonly lies from four to six feet deep. The Andes moun

tains, though part of them are situated in the torrid zone, are at the summit covered with snow, which cools the air in the adjacent country. The heat on the western coast of Africa, after the wind has passed over the sandy desert,is almost suffocating; whilst that same wind having passed over the Atlantic Ocean, is cool and pleasant to the inhabitants of the Caribbean Islands.

I. CLIMATES between the equator and the POLAR CIRCLES.

II. Climates between the POLAR CIRCLES and the POLES.

The preceding tables may be constructed by the globe, as will be shewn in the problems, but not with that exactness given above. Tables of this kind are generally copied from one author into another, without any explanation of the principles on which they are founded.

tor

Construction of the first Table.

In plate IV. figure IV. HO represents the horizon, ÆQ the equàa parallel of the sun's greatest declination, NO the elevation of the pole or latitude of the place; the angle c a b, measured by the arch Q O, the complement of the latitude; a b is the ascensional difference, or the time the sun rises before 6 o'clock, and b c the sun's declination. Hence, by Baron Napier's rules (see Keith's Spherical Trigonometry) rad.Xsine a bcotangent a* (or tangent NO) Xtangent b c.

VIZ. As tangent of the sun's greatest declination 230 28',
Is to radius, sine of 90 degrees;

So is sine of the sun's ascensional difference,
To tangent of the latitude- -A general rule.

At the end of the first climate the sun rises before 6; and in every climate, if you take half the length of the longest day, and deduct 6 hours therefrom, the remainder turned into degrees will give the ascensional difference. Hence, the ascensional difference, for the first climate, is 15 minutes of time, equal to 30 45'; for the second climate 30 minutes

11° 15'; for the fourth As tangent of 23° 28'

Is to radius, sine of 90°
So is sine of 3° 45'

To tangent lat. 8° 34′

7° 30' for the third climate 45 minutes climate 1 hour 15°, &c. 9.63761 As tangent of 23° 8'

9.63761

10.00000 is to radius,sine of 90o 10 00000 8.81560 So is sine of 7° 30'

9.11570

9.17799 To tangent lat. 16° 44' 9.47809

Construction of the second Table.

The longest day is the 21st of June, when the sun's declination is 23° 28' north. Count half the length of the day from the 21st of June forward and backward; find the sun's declination answering to those two days in the nautical almanac, or in a table of the sun's declination; add the two declinations together, and divide their sum by 2, subtract the quotient from 90 degrees, and the remainder is the latitude. As the sun's declination is variable, it ought to be taken out of the almanac, or tables, for leap year and the three following years; a mean of these declinations used as above will give

D

the latitude as correct as the nature of the problem admits of, and in this manner the second table was constructed.-Riccioli (an Italian astronomer and mathematician, born at Ferrara, in the Pope's dominions, 1598,) in his Astronomia Reformata, published in 1665, makes an allowance for the refraction of the atmosphere in a table of climates. He considers the increase of days to be by half hours, from 12 to 16 hours; by hours, from 16 to 20 hours; by 2 hours from 20 to 24 hours; and by months in the frigid zones, making the number of the days of each month in the north frigid zone, something more than those in the south; but, as the refraction of the atmosphere is so extremely variable that scarcely any two mathematicians agree with respect to the quantity, it is evident that a table of climates, calculated with such an uncertain allowance, can be of no material advantage.

70. A ZONE is a portion of the surface of the earth contained between two small circles parallel to the equator, and is similar to the term climate, for pointing out the situations of places on the earth, but less exact; as there are only five zones, whereas there are 60 cli

mates.

71. The TORRID ZONE extends from the tropic of Cancer to the tropic of Capricorn, and is 46° 56' broad. This zone was thought by the ancients to be uninhabited; because,it is continually exposed to the direct rays of the sun; and such parts of the torrid zone as were known to them were sandy deserts, as the middle of Africa, Arabia, &c.; and this sandy desert extends beyond the left bank of the Indus, towards Agemere.But these deserts are not produced merely by the excessive heat of the sun, as the ancients imagined; because,it is well known, that moisture is one of the greatest inconveniences in several parts of the torrid zone:

72. The two TEMPERATE ZONES. The north temperate zone extends from the tropic of Cancer to the arctic circle; and the south temperate zone from the tropic of Capricorn to the antarctic circle. These zones are each 43°4′ broad, and were called temperate by the ancients, because, meeting the sun's rays obliquely,they enjoy a moderate degree of heat.

73. The two FRIGID ZONES. The north frigid zone, or rather segment of the sphere, is bounded by the arctic circle. The north pole which is 23°28′ from the arctic circle, is situated in the centre of this zone. The south frigid zone is bounded by the antarctic circle, distant 23° 28′ from the south pole, which is situated in the centre of this zone.

74. AMPHISCII are the inhabitants of the torrid zone; so called, because they cast their shadows both north and south at different times of the year; the sun being some times to the south of them at noon, and at other times to the north. When the sun is verticle, or in the zenith, which happens twice in the year, the inhabitans have no shadow, and are then called Ascii,or shadowless.

75. HETEROSCII is the name given to the inhabitants of the temperate zones, because they cast their shadows at noon only one way. Thus, the shadow of an inhabitant of the north temperate zone always falls to the north at noon, because the sun is then directly south; and an inhabitant of the south temperate zone casts his shadow towards the south at noon, because the sun is due north at that time.

76. PRIESCII are those people who inhabit the frigid zones, so called, because their shadows, during a revolution of the earth on its axis, are directed towards every point of the compass. In the frigid zones the sun does not set during several revolutions of the earth upon its axis.

77. ANTOECI are those who live in the same degree of longitude, and in equal degrees of latitude, but the one has north, and the other south latitude. They have noon at the same time, but contrary seasons of the year; consequently, the length of the days to the one, is equal to the length of the nights to the other. Those who live at the equator have no Antœci.

78. PEIIOECI are those who live in the same latitude, but in opposite longitudes; when it is noon with the one, it is midnight with the other; they have the same. length of days, and the same seasons of the year. The inhabitants of the poles have no Periœci.

79. ANTIPODES are those inhabitants of the earth who live diameterically opposite to each other, and, consequently, walk feet to feet; their latitudes, longitudes, seasons of the year, days and nights, are all contrary to each other.

80. The RIGHT ASCENSION of the sun, or a star, is that degree of the equinoctial, which rises with the sun, or a star,in a right sphere,and is reckoned from the equinoctial point Aries eastward round the globe.

81. OBLIQUE ASCENSION of the sun, or a star, is that degree of the equinoctial which rises with the sun, or a star in an oblique sphere, and is likewise counted from the point Aries eastward round the globe.

82. OBLIQUED SC NSION of the sun or a star, is that degree of the equinoctial which sets with the sun or a star in an oblique sphere.

83. ASCENSIONAL or DESCENSIONAL DIFFERENCE,is the difference between the right and oblique ascension, or the difference detween the right and oblique descension, and with respect to the sun, it is the time he rises before 6 in the summer, or sets before in the winter.

84. The CR PUSCULUM, or TWILIGHT, is that faint light which we perceive before the sun rises,and after he sets. It is occasioned by the earth's atmosphere refracting the rays of light, and reflecting them from the particles thereof. The twilight is supposed to end in the evening, when the sun is 18 degrees below the horizon, or when stars of the sixth magnitude (the smallest that are visible to the naked eye) begin to appear; and the twilight is said to begin in the morning, or it is daybreak, when the sun is again within 18 degrees of the horizon. The twilight is the shortest at the equator, and longest at the poles; here the sun is near two months before he retreats 18 degrees below the horizon, or to the point where his rays are first admitted into the atmosphere; and he is only two months more before he arrives at the same parallel of latitude.

85. REFRACTION. The earth is surrounded by a body of air called the atmosphere, through which the rays of light come to the eye from all the heavenly bodies; and since these rays are emitted through a vacuum,or at least through a very rare medium, and fall obliquely upon the atmosphere, which is a dense medium, they will, by the laws of optics, be refracted in lines approaching nearer to a perpendicular from the place of the observer (or nearer to the zenith) than they would be were

Any fluid, or substance, through which a ray of light can penetrate, is called a medium, as air, water, oil, glass, &c. The air near the surface of the earth is more dense than in the higher regions of the atmosphere; and beyond the atmosphere the rays of light are supposed to meet with little or no resistance.