Or, the conversion can be made by Table XII., but not

so readily.

To convert Time into Degrees of Longitude (or Arc).

Rule.-Express the hours and minutes as minutes only, and divide by 4; then, the minutes of time become degrees; the seconds of time become minutes (') of arc, &c.

Similarly, 20m. 49s. 5° 12′ 15"; and 10h. 15m. 42s. 153° 55′ 30′′.

Time may also be converted into arc by multiplying by 15, or its equivalent, 3 times 5; when seconds of time become" of arc, minutes of time' of arc, and hours become degrees. Thus

Or the conversion may be made by Table XII. We are now in a position to understand (1) the relation of civil to astronomical time; and (2) the relation of ship time and longitude to the Greenwich Date.

TO CONVERT CIVIL TIME INTO ASTRONOMICAL TIME.

If the civil time is P.m. (after noon), meaning hours between noon and midnight, it corresponds to the astronomical time; thus—

April 8th, at 7h. 26m. P.m. (civil time), is April 8d. 7h. 26m. (astronomical time).

If the civil time is A.m. (before noon), meaning hours between midnight and noon, add 12h. to the civil time, and put the date 1 day back; thus—

April 8th, at 7h. 26m. A.m. (civil time),

is April 7d. 19h. 26m. (astronomical time). Also, November 1st, at 10h. 46m. A.m. (civil time), is October 31d. 22h. 46m. (astronomical time).

THE TIME AT SHIP AND LONGITUDE BEING GIVEN, TO FIND THE GREENWICH DATE.

1. Express the ship date as astronomical time. 2. Convert the longitude into time, and place it under the ship time.

3. If the longitude is West, add it to the ship time. (Note.-If the sum of the hours is more than 24h., reject 24h. and add 1 day to the date.)

4. If the longitude is East, subtract it from the ship time. (Note.—If the hours of longitude exceed the hours at ship, add (mentally) 24h. to the ship time, and put the date 1 day back.)

TO FIND OUR LATITUDE.

This is to find our distance from the equator. That point of the heavens which is immediately over our heads is termed the zenith; and at some spot between the tropics the sun at noon is immediately or nearly overhead; but as we move farther North or South, it becomes more distant from our zenith; so that when we know what this distance may be, the sun's distance from the equator, we at once know our distance from the equator, that is, our latitude. To ascertain the Zenith Distance, we thus proceed. If two imaginary lines are drawn from our feet, one to our zenith, the other to the horizon, they will be at right angles to each other, that is, the two lines at their junction will form an angle of 90°. If, again, we imagine a third line to be drawn from our feet to the sun's centre, and are enabled to learn the number of degrees contained between this last line and the one drawn towards the horizon (which the quadrant or sextant will do for us), when we subtract these degrees from 90°, the remainder will be the number of degrees contained between the imaginary lines from our feet to the sun and from our feet to the zenith; or, in other words, the number of degrees the sun may be from the zenith. Thus, suppose the number of degrees contained by the lines from our feet, one to the sun and the other to the horizon, to be 30, then this number being subtracted from 90°, will leave 60° to be contained by the lines from our feet to the sun and zenith respectively; that is, the sun's zenith distance is 60°.

The number of degrees contained by the lines from our feet, one to the sun and the other to the horizon, denote the sun's altitude above the horizon. We now set about to find out the amount of this angle, or the

sun's altitude; subtract it from 90°, and the zenith distance becomes known.

The Sun's Declination is the distance of the sun from the equinoctial line, which is an imaginary line in the heavens coincident in space with the earth's equator; therefore, when we know the distance of our zenith from the sun, and that of the sun from the equinoctial line, as this last is exactly over the equator, we know our distance from it, or our latitude.

During our summer, the sun is to the north of the equinoctial line, and in the winter to the south of it. This is termed its North or South Declination, and tables are computed which give us the amount of this distance or declination for every day and hour throughout the year.

To correct the Sun's Declination for finding Latitude by the Meridian Altitude.—In the Nautical Almanac the sun's declination at noon is put down, but only for the meridian of Greenwich Observatory; therefore at any place considerably to the east or west of this meridian, allowance must be made for this difference of longitude. On taking out the sun's declination from the Nautical Almanac, note whether the declination is North or South, increasing or decreasing; thus—August 2G, 1882. Sun's Decl". 10° 22' 16" N. decreasing—Longitude in 92°. Turn to Table XXIX., and enter it as directed in the explanation, and 5' 30" is the correction to be added or subtracted, for the difference of longitude, as directed.

But by the side of the Declination in the Naut. Alm. you will see a column headed "Var. in one hour;" this is the hourly change of the declination, which, if multiplied by the Long. in time, must give us the required declination for the position; hence—

Rule.—From Naut. A1m., p. I., of month, take out the Declination for the Ship Date, and also the "Var. in 1 h." for the same date. Multiply the "Var. in 1 h." by the Longitude in Time, and the product will be the correction, to be applied as follows:

In W. long.

In E. long.

Dec. decreasing, subtract correction.
[Dec. increasing, add correction.
Dec. increasing, subtract correction.
Dec. decreasing, add correction.

Ex. Same date: Long. 36° W.

Ex. August 26th, 92' E.

1882: Long.

36°
4

Var. 52" 25

2.4

20900

10450

6,0) 12,5.400

2 5.4

10 22 15.6

Red. Dec. 10 20 10.2 N.

Note.-The Dec. is corrected to the nearest tenth of an hour; thus 8 m. is taken as 1, and 24 m. is of course 4 of the hour.

This is the best method of getting the declination, and the result is called the Reduced Declination. Such is the first step in finding our Latitude from a Meridian Altitude, or from the sun's altitude at noon.

As soon as we have ascertained, by means of the quadrant, the Altitude, we have several modifications to apply to it: it is termed the Observed Altitude, in contradistinction to the True Altitude of the sun's centre above the horizon.

We take the altitude of the sun's lower limb because it can be brought apparently to touch the horizon to great accuracy; whereas if we were to place the sun's centre in junction with the horizon, this centre would have to be guessed at.

The Sun's observed altitude and its corrections.—The altitude of the sun's lower limb above the horizon hav