Sidebilder
PDF
ePub

notched, or drawn with fmall Lines, the Glafs is veiny, and must be rejected; if more Images than one of the fame Object are feen, it fhews that the two Surfaces are not ground Parallel; the other Speeulum may be examined in the fame Manner.

Obferve the Sun or a Candle through the dark Glaffes feverally, holding the Glafs about eight or ten Inches from the Eye; if they are veiny the Object will appear notched at the Edges, but if clear and well defined, the Glaffes are good:

Moft People prefer black Ebony, on Account of its Weight, but I have found by Experience, that good Mahogany takes the Glue, and ftands the Heat better.

Quadrants, like Watches, may appear well to the Eye, and yet be good for little; it is therefore much better to give two Guineas and an Half, or three Guineas, for a good one, that will laft a Man for Life, than purchase those wretched Inftruments made up at a low Price, which cannot be depended on.

[blocks in formation]

The Rays of Light in paffing through the Atmosphere are bent out of their ftraight Courfe into a curve Line; and hence it happens that all the Heavenly Bodies, except when they are in the Zenith, appear higher than they ought to do, and so much the more the nearer they are to the Horizon. This apparent Elevation of the Heavenly Bodles above their true Height is called the Refraction of Objects; and the Quantity or Effect of it, according to the different Altitudes of Objects, has been carefully obferved by eminent Aftronomers, and must always be fubtracted from the Apparent Altitude, but added to the apparent Zenith Distance of an Object with whatever Inftrument the Obfervation is made, in order to obtain its true Altitude of Zenith Distance.

That the Corrections of obferved Altitudes of Objects, both on Account of the Dip of the Horizon and the Refraction of their Light, may appear at one View, they are both exhibited together in Tables and 6.

5

To work an Obfervation, or to find the Latitude of a Place by the Tables of the Sun or Star's Declination and the Zenith Diftance.

The Latitude of any Place is its Distance from the Equator, either North or South, counted in Degrees, &c. upon an Arch of the Meridian, contained between the Zenith, or that Point directly over your Head and the Equator. It can never exceed 90 Degrees, and is found by taking the Altitude or Height of the Sun or Star above the Horizon of the Sea with a Quadrant, when on the Meridian or due North or South of the Place of Obfervation.

This Meridian Altitude, corrected for Dip of the Horizon; and Refraction, and 16 Minutes the Sun's Semi-diameter added thereto, gives the Altitude of his Centre, which being fubtracted from 90° gives the Zenith Distance or the Number of Degrees, &c. the Cen

tre of the Object is from the Point over your Head; with which, and knowing how far the Object is to the North or South of the Equator, which is called Declination, the Latitude is found by the Meridian Altitude of any celestial Object as follows:

Firft, If the Sun or Star be South when obferved, call the Zenith Distance South; but if North, call it North.

Then if the Zenith Distance and Declination be of contrary Names; that is, if the Sun or Star comes to the Meridian in the North, and hath South Declination, or to the South, and hath North Declination, the Zenith Diftance, added to the Declination, gives the Latitude of the Place of Obfervation; and of the fame Name the Declination is of, whether North or South.

Secondly, When the Zenith Distance and Declination are of the fame Name; that is, when the Sun or Star comes to the Meridian in the North, and hath North Declination; or to the South, and hath South Declination, fubtract the Leffer from the Greater, and the Remainder is the Latitude; and to know whether it is North or South to observe this general Rule:

When the Declination is greater than the Zenith Distance, the Latitude is of the fame Name with Declination; but if lefs, of a contrary Name.

NOTE. First, When the Sun or Star is on the Equator, or hath no Declination, the Zenith Distance is the Latitude of the Place, and of a contrary Name to the Zenith Distance.

Secondly, When the Sun or Star is in the Zenith, the Declination is the Latitude, and of the fame Name as the Declination is of. For it is evident, that as they are equally diftant from the Equator, and on the fame Side of it, confequently if the Declination be North, the Latitude will be alfo North, and if South, South.

[blocks in formation]
[blocks in formation]

Equator.

NOTE. The Declination here is fitted to the Year 1796. The foregoing Rules are for obferving by the Sun or Stars when they are at their greateft Altitudes, or upon the Meridian above the Pole, but as in fome Parts of the Earth, the Sun does not fet for feveral Days, and fome Stars never fet, in that Cafe they may be obferved upon the Meridian twice in 24 Hours, that is, once at their greateft Height as before, and again when they are at their loweft or upon the Meridian below the Pole, to work which Obfervations, take the following

RULE. Add the Complement of the Declination to the Meridian Altitude, the Sum is the Latitude of the fame Name with the Declination.

[blocks in formation]

At Sea I took the Altitude of the North Pole Star, when on the Meridian below the Pole, and found it 46° 21′, required the Latitude?

[blocks in formation]

In the following Examples the Corrections for Dip and Refraction are introduced.

NOTE. As all the Calculations are for 1796, thofe that teach by this Book, will do well to furnith themselves in time with the above Almanack.

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

By a back Obfervation with Hadley's Quadrant, the App. Alt. of the Sun's lower Edge was 258 12' S. when his Declination was 21° 14' S. and the Eye 40 Feet above the Horizon, in what Latitude was the Obfervation made?

Obf. Alt. Sun's lower Edge 25° 12' S.
Semi-diam. to be fubtracted 16

App. Alt. Sun's Centre
Dip of Hor. to be added

24 56 об App. Alt. correct by the Dip 25 02 Refraction to be fubtracted

True Alt. Sun's Centre True Zenith Distance Declination

Latitude

02

25.00

65 30 S. 21 14 S.

43 46N.

20 04 34 N.

EXAMPLE. XI.

Suppofe on the 12th June 1796, an Obferver in an high Northern Lat. and 65" Weft of London, his Eye being 28 Feet high, obferved the Alt. of the Sun's lower Limb on the Meridian below the Pole to be 15′Es by a fore Obfervation with Hadley's Quadrant. Required the Latitude?

The Sun being obferved below the Pole, it must have been at 12 Hours paft Noon at the Place of Obfervation, and that Place being 65° W. of London 4 Hours 20' later than at London, therefore it must have been 16h. zo' past Noon at London.

*June 12, 1796, the Sun's Declination 23° 13′ 46′′ the daily Variation is 312": And as 24h. 3′ 12′′ :: 16h. 20: 2' 11', which added to 23o 13', 46" (because the Declination is in creafing) gives 23° 16', the Sun's Declination at the Time and Place of Obfervation nearly.

* See Nautical Almanack for 1796.

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

THE of the Place and the Magnetic HE Variation of the Compass is an Arch of the Horizon conMeridian, and is either Eaft or Weft; or it is the Number of Dethe Needle's Point ftands from the true North or South Points of the Horizon, reckoned to the Eastward or Westward, and is readily found either from the Sun's Amplitude or Azimuth.

To find the true Amplitude.

The Sun's true Amplitude is an Arch of the Horizon comprehended between the true Eaft or Weft Points thereof, and the Cen tre of the Sun at its rifing or Setting; or it is the Number of Degrees, &c. the Sun rifes or fets to the Northward or Southward of the Eaft or Weft Points of the Horizon.

The Sun's Magnetic Amplitude is the Number of Degrees, &c. the Centre is Northward or Southward of the Eaft or Weft Points of the Compafs at his Rifing or Setting, and is found with an Azimuth Compafs in the following Manner :

Move the Box about till the four Cardinal Points of the Card coincide with the four Lines in the Sides of the Compafs-Box; Keep the Box fteady, and turn the Index till the Centre of the Sun at Rifing or Setting, when his lower Edge just touches the Horizon is feen through both the Sights; then the Degrees between the North Point of the Compafs, and the End of the Index pointing to the Sun, will be the Magnetic Amplitude given.

I

The

« ForrigeFortsett »