add

12 00

Correction from Table XVI. 3h 39′ 50′′ bef. noon on the 26th, sub.

Correc. from Table XVI. for long. 45° E.

2 29

0.01199 Correct declination, north,

9.64713

Half sum, equal co-sine 63 39 26, half hour angle,

2

The correction of the sun's altitude, Table V. is the difference between its refraction and parallax in altitude.

On the Planets.

THE planets Jupiter, Mars, and Saturn, are very serviceable for obtaining the latitude. The planets are easily distinguished from the fixed stars by their steady light, as they never, like the latter, twinkle, except when very near the horizon. Mars may be known by his ruddy complexion; Jupiter commonly appearing large and refulgent. Saturn is of a pale cast and feeble light, sometimes resembling a star of the first or second magnitudes. Their declinations, and time of their passing the meridian of Greenwich is given in the Nautical Almanac in page V. of the month, for every six days, and may be proportioned to any intermediate time.

No dependance must be put in the altitude of either stars or planets without a distinct horizon. These planets, given in the Nautical Almanac, and the fixed stars given in the epitomes of navigation, and the moon, may be of the greatest importance for ascertaining the latitude from their meridian altitudes, as in high latitudes it is frequently hazy in the day-time, and clear at night. About the Cape Verd islands, being frequently hazy in the day, and clear at night, the use of the above planets for finding the latitude by their meridian altitude may be of material service. In the ship Recovery of Philadelphia, in 1815, being bound to the Cape Verd islands, when drawing up with them, I was for several days, by the haze, prevented from obtaining the sun on the meridian; but at night, it being clear, I obtained the latitude by the planet Mars; and I ran down the latitude of the Isle of May with the latitude obtained by the meridian altitude of the above planet, and made it as I wished.

See rules for finding the latitude by the meridian altitude of a planet in Bowditch's American Practical Navigator.

To find the Longitude by an Eclipse of the Moon.

AT the times when the eclipse begins and ends at Greenwich,* observe times when it begins and ends at any other

The time of the eclipses of the moon on the meridian of Greenwich is given in the Nautical Almanac.

place. The difference of these times, converted into longitude, will give the longitude of the ship or place.

For the above purpose a watch should be previously regulated by the sun's altitude, or the error found and allowed for.

An eclipse of the moon, arising from its real deprivation of light, must appear to begin at the same instant to every place on that part of the earth which is turned towards the moon. The moon enters the penumbra of the earth before it comes to the umbra, and therefore it gradually loses its light, and the penumbra is so dark at the beginning of the umbra, that it is difficult to ascertain the exact tiine when the moon's limb touches the umbra, or when the eclipse begins.

When I observed an eclipse of the moon, for the purpose of obtaining the longitude, as I could not distinguish between the penumbra and umbra, therefore, when the faint shadow or penumbra first touched the moon's limb, I allowed thirty seconds to sixty seconds of time, according to my judgment, before I marked the time. This destroyed a part of the error. In this way the longitude may be obtained, within from fifteen to sixty miles.

The penumbra is a faint or partial shadow, observed between the perfect shadow and the full light, in an eclipse; and this degree of light and shadow will be greater or less as the point lies open to a greater or less part of the sun's body.

On Quadrants.

QUADRANTS should have three screens (glasses behind the horizon glass) such as are fixed to a sextant. They will be found useful in taking the sun's altitude when low, to take his glare off the horizon, also when taking an altitude of the sun for the purpose of obtaining the apparent time, or for obtaining an azimuth, or if the time should be wanting on shore. The sun's altitude may be observed in an artificial horizon of quicksilver, tar, or molasses, or in a bucket or basin of water; and the index error may also be obtained by having these three additional glasses; otherwise not.

In high latitudes, when observing the altitude of sun or star, to ascertain the apparent time, it is necessary to have the latitude correct, particularly if the latitude and declination are of contrary names; that is, if the one be north and the other south; as a small error in the latitude will make a considerable error in

the apparent time. When it is suspected that there is much error in the latitude, it is best, after you have worked for the apparent time, to work for it again in one degree different from the former. By this means you will know how much you can depend on the apparent time. It is best to observe the altitude of the object when it is in the prime vertical, that is when it bears due east or due west; but if the declination of the object should be of a contrary name to the latitude of the ship, it cannot appear in the prime vertical. In this case the altitude should be observed as near the horizon as possible, but not less than three degrees high, on account of the variation of refraction.

See Table XV.

On Marking a Log-line.

SIX thousand feet being one sea mile, the sixtieth part of which is one hundred feet; and one minute being the sixtieth part of an hour, it follows, that if a log-line were marked at one hundred feet to the knot, that the glass should be one minute long. As, if a ship goes one hundred feet in a minute, she will go six thousand feet in an hour. But the usual way now is, to mark the line forty-eight feet to the knot, and to make use of a twenty-eight second glass.

On the Importance of having a good Watch.

A GOOD watch is of great service on board of a ship at sea. In cloudy weather, when the sun is occasionally seen, frequently only a single sight of his altitude can be obtained. He being immediately obscured by clouds after this observation, it cannot be known by it, whether he is on the meridian or not; and therefore the observation is rejected. But, if you have a good watch, which has been previously regulated by an observation of the sun's altitude, taken at a time when he was at least three hours from the meridian, it will show the apparent time; and you may thereby know how far your observation may be depended on.

In the case, also, where an observation of the sun cannot be obtained when he is on the meridian, from the interception of clouds, but is taken a little time before or after meridian, a watch, thus regulated, is also very useful. With respect to this the

method which I adopted is the following:-On a clear day I observed the sun five, ten, fifteen, and twenty minutes before or after he came on the meridian, as well as at meridian, and noted down the difference between the altitude at these times and the meridian altitude. When, therefore, I could not get an observation when the sun was on the meridian, but observed him within any of the times above noticed, I allowed the differences which I had previously observed these times made in the altitude, compared with the meridian altitude.

It must be observed, however, that this will answer only in high latitudes.

On the Precession of the Equinoxes.

THE precession of the equinoxes has been already mentioned; the cause of which I shall here mention according to Mr. Ferguson. Here the length of the solar year differs a little from Mr. Gregory.

By the earth's motion on its axis, there is more matter accumulated all around the equatorial parts than any where else on the earth.

The sun and moon, by attracting this redundancy of matter, bring the equator sooner under them in every return towards it, than if there was no such accumulation. Therefore, if the sun sets out as from any star or other fixed point in the heavens, the moment when he is departing from the equinoctial or from either tropic, he will come to the same equinox or tropic again twenty minutes seventeen and a half seconds of time, or fifty seconds of a degree, before he completes his course, so as to arrive at the same fixed star or point from whence he set out. For the equinoctial points recede fifty seconds of a degree westward every year, contrary to the sun's annual progressive motion.

When the sun arrives at the same equinoctial or solstitial point, he finishes what we call the tropical year, which, by observation is found to contain three hundred and sixty-five days, five hours, forty-eight minutes, fifty-seven seconds; and when he arrives at the same fixed star again, as seen from the earth, he completes the sideral year, which contains three hundred and sixty-five days, six hours, nine minutes, fourteen seconds and a half. The sideral year is therefore twenty minutes seventeen seconds and a half longer than the solar or tropical year, and nine minutes fourteen seconds and a half longer than the Julian or civil year, which we state at three hundred and sixty-five days six