The degree is usually reckoned in round numbers, at 691 miles; but if accuracy be attended to, the number in the table is too large : the real length of a degree is 365184 English feet, or 69 miles 288 yards this has been ascertained by actual measurement, so that the circumference of the earth is equal to 69 miles, 288 yards × 360 (because in every circle there are 360 degrees) = 25000 miles nearly.

Geographers reckon on the globe two kinds of degrees, viz. degrees of latitude, and degrees of longitude.* The degrees of latitude, which are measured, from north to south, on the meridian, are all of one length, as above. But the degrees of longitude, or the circles which pass round the earth in each parallel of latitude, continually diminish in proceeding from the equator towards the Poles, but at the equator they are of the same length as those of latitude. The following is a Table of the length of the degrees of longitude, carried to three places of decimals, in every 5 degrees of latitude.

Here it is evident, that at latitude 40°, the degree is little more than 53 miles in length; at 70' it is only 23 miles; and at the pole, or 90°, it comes to nothing, it being supposed to be a point.

I. To find the distance, in miles, between any two places, having the same degree of latitude.

RULE. Having found the distance between the places, in degrees, multiply the number so found by the number in the table opposite the given degree of latitude.

Ex. How many miles distant is Madrid, in Spain, from Bursa, in Natolia; the latitude of both is 40° Ñ., but the long. of Madrid is about 3° W., and that of Bursa 29° E.?

NOTE.

*Longitude expresses the distance of meridians, or circles, which are supposed to pass over the head from north to south; and latitude expresses the distance of a place north and south from the equator.

The difference in longitude is 3° + 29° — 32o, this multiplied by 53.01, the number of miles in a degree at the given latitudes, gives 1696 for the miles between Madrid and Bursa.

II. To find the distance between any two places, having the same degree of longitude.

RULE. Multiply the number of degrees between the places by 69.2, and the answer is in miles.

Ex. How far is London from Mount Atlas in Africa, the former is 51° N. L., the latter 311° N. L.?

360°

Here the distance is 20°, and 20 × 69.2 1384 miles. TIME is measured by the revolution of the earth about its axis: every revolution is completed in 24 hours; and as there are 360° in the great circle of the earth, so 15° 1 hour of time.-Hence this TABLE: 15° of motion answers to 60' in time, or 1 hour.

24

=

RULE. Multiply the hours by 15, and divide the minutes by 4, and the answer is in degrees, &c.

Thus 4 h. 20 min. in time, answer to 65° in motion.

II. To convert motion into time.

RULE. Divide the given number of degrees of motion

by 15, and the answer is in time.

Thus 65° of motion answer to 4 h. 20 inin. in time:

15)65(4

60

5

60

15)300(20

300

Ex. 1. What o'clock is it at Athens, which is 23° 57′ east longitude of London, now it is 12 at the metropolis? Athens being east of London, the clocks there will be before the clocks here. 15)23° 57′(1 hour.

15

8

60

Answer. When it is 12 o'clock at London, it will be 36 min. past 1 at Athens.

15)537(36 min. nearly.

Ex. 2. What o'clock is it at Philadelphia in America, now it is 12 at London ?

Philadelphia is 75° 8' west longitude of London, of course the clocks there are behind those here.

15)75° 8'(5 ho. 0 min. 32 sec.

75

8

60

15)480(32

In this case the answer is.

12 h. 5 h. 0 m. 32 s. 6 h. 59 m. 28 s. ́or very nearly 7 in the morning.

In many maps the longitude is reckoned from Ferro, one of the Canary Islands, which is 17° 45' west of London. III. To reduce the longitude of Ferro to that of London. RULE 1. If the place be EAST of London, subtract from it 17° 30′, and the remainder is the longitude east of London. Thus, from Ferro, Constantinople is 46° 44′; to reduce this to the longitude reckoned from the meridian of London, we say,

46° 44' - 17° 45′ 28° 59′.

2. If the place be WEST from Ferro, add to the given longitude 17° 45'.

Thus, Boston is 52° 48′ west of Ferro, but it is west of London 52° 48′+17° 45′ = 70° 33'.

3. If the place lies between Ferro and London, its longitude will be obtained by subtracting its longitude east of Ferro from 17° 45'.

Thus, Lisbon is 8° 40′ east of Ferro, and it is west of London 17° 45′ — 8° 40′ = 9o 5'.

By a reverse method may be reduced the longitude from London to that of Ferro.

The earth being globular, it is a useful problem to ascertain the extent of the visible horizon: or

IV. To find the distance to which a person can see at any given height of the eye.

RULE. Multiply the square-root of the height of the eye, in feet, by 1.2247, and the product is the distance in miles to which we can see from that height. See p. 143.

Ex. 1. How far can a sailor see, standing at the topmast of a ship, 144 feet high?

The square-root of 144 is 12; therefore 1.2247 X 12 14.7 miles. Thus, in this situation, a sailor might, on a very clear day, descry land at the distance of 5 leagues, nearly; and he might see the top-mast of another ship at a still greater distance.

Ex. 2. To what distance could a person see from the top of St. Paul's, which is 340 feet high?

340 × 1.2247 more than 22 miles.

18.44 X 1.224722.58 miles, or something