Sidebilder
PDF
[ocr errors]

What will be the Moon's true Altitude at 19 H. 16 M. 52 S. apparent Time, in Latitude 14° 45' S. and Longitude 167° E. on August 24, 1796 :

[ocr errors]
[merged small][ocr errors][ocr errors][merged small][ocr errors][ocr errors]

In the last Example proportional Parts are taken in finding the Right Ascension, Declination and Log. Rising. By the three last Cases the true Altitudes of the Obječis are found, therefore if the apparent Altitudes be wanted, the Difference between the Sun’s Parallax and Refračtion must be added to the Sun’s true Altitude, the Refračtion must be added to the true Altitude of a Star, and the Difference between the Moon's Refračtion and Parallax in Altitude must be subtracted from the true Altitude of the Moon thus found, to obtain the respective apparent Altitudes of their Centres.

To find the Longitude by the Eclipses of Jupiter's Satellites.

On the Day preceding the Evening on which it is proposed to observe an Eclipse, look for the Time when it will happen at Greenwich, in Page 3d of the Month in the Ephemeris. Find the Diff. of Longitude, either by a good Map, Sea Chart, or Dead ReckonLet the Watch be regulated by the Sun with all possible Exačtness to the apparent Time. Turn the Difference of Longitude into Time, and add it to or subtract it from the apparent Hime, according as it is East or West of Greenwich, the Sum or Difference will be nearly the Time when the Eclipse is to be looked for in that Place. But as the Longitude is uncertain, it will be proper to begin 20 or 30 Minutes before. Observe the Hours, Minutes and Seconds of the beginning of the Eclipse, called Immersion, that is, the very Instant that the Satellite appears to enter into theShadow of Jupiter; or the Emersion, that is, when it appears to come out of the same. The Difference of Time between the observed Immersion, or Emersion, and that set down in the Nautical Almanack, being turned into Degrees, will give the Difference of Longitude between Greenwich and the Place of Observation. These Observations made on the first Satellite, or that which moves nearest to the Body of Jupiter, is the most proper for determining the Longitude; and here it may be observed, that its Emersions are not visible from the Time of Jupiter’s Conjunction with the Sun to the Time of his Opposition to the Sun, and that its Immersions are not visible from the Time of the Planet's Opposition to the Sun, to the Time of its Conjunction. The Configurations, or the Positions in which Jupiter’s Satellites appear at Greenwich, are laid down every Night when visible, in Page the 12th of the Month in the Ephemeris.

ing. - Let

[ocr errors][ocr errors][ocr errors][ocr errors][ocr errors]

To find the Longitude by the Eclipses of the Moon.’

This is performed by comparing the Times of the Beginning or

Ending, as abso the Times when any Number of Digits are eclipsed, or when the Earth's Shadow begins to touch, or leave any remarkable Spot on the Moon's Face. -

Then will the Difference of Time between the like Observations made at different Places, turned into Degrees, be their Difference of Longitude. .

But these Eclipses happen too seldom to be of any general Use at Sea.

The Manner of Surveying Sea Coasts and Harbours. *

I AVING, in the former Part of this Work, treated on those Branches of Knowledge which ought to be acquired by every one who undertakes the condućting of Ships to remote Parts; I think it incumbent on me to give some Directions concerning another Branch, which, though of great Importance, seems to be too much neglečted; namely, the drawing of Draughts or Charts of the

[ocr errors]
[ocr errors]
[ocr errors]
[ocr errors][ocr errors][ocr errors][ocr errors]
[ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][merged small]
[ocr errors]

runnin

Way,

out of the Log-line, and to keep an Account of the Ship's

[ocr errors]

f the Lead, to write down the

ne or two Head Points, or remarkable

[ocr errors]

rings of o

o as to be readily able to tell the Distance run when required

the third to attend the Heav
Parts of the Coast taken at each De
man, to draw out the necessar
neate the Figure and Windin

Soundings and Be

[ocr errors]
[ocr errors]
[ocr errors]

correč

« ForrigeFortsett »