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Chambers, Ephraim, 1680 (ca.)-1740 / Cyclopædia, or, An universal dictionary of arts and sciences : containing the definitions of the terms, and accounts of the things signify'd thereby, in the several arts, both liberal and mechanical, and the several sciences, human and divine : the figures, kinds, properties, productions, preparations, and uses, of things natural and artificial : the rise, progress, and state of things ecclesiastical, civil, military, and commercial : with the several systems, sects, opinions, &c : among philosophers, divines, mathematicians, physicians, antiquaries, criticks, &c : the whole intended as a course of antient and modern learning
(1728)

Locustae - lysiarcha,   pp. 466-477 PDF (10.9 MB)


Page 471


(471 )
Difference of thofe Times gives that of the Longitude of
thofe Places. Now in theEpbemerides, we have the Mo-
tions of the Planets, and the Tines of all the Celeflial
.bhenomena, as the Beginning and Ending of Eclipfes,
Conjunaions of the Moon with other Planets, its En-
trance into the Ecliptic, &c. accurately calculated for
fome one Place. Therefore if the Hour and Minute be
known wherein any of the fame Phenomena are obferv'd
in an unknown Place, the Difference between the Hour
and Minute between that Place and that other, to which
the Tables are calculated, and confequently the Diffe-
rence of their Meridians, and their Longitude from each
other, are known alfo. Now the Difficulty, here, does
not confifi in the exad finding of the Time, which is ea-
fily had from the Sun's Altitude or Azimuth; but the
Defea lies in the Paucity of proper Appearances capable
of being thus obferv'd: For all flow Motions (v.g. that
of Saturn) are at once excluded; as Jhewing but little
Difference in a confiderable Space of Timee; and it being
here required that the PbTenomenon be fenfibly varied in
two Minutes Time, an Error of two Minutes in Time pro-
ducing another of 30 Miles in the Longitude. Now there
are no Phenomena in the Heavens that have there Requi-
fites, excepting the feveral Stages of an Eclipfe of the
Moon; her Longitude, or Place in the Zodiac; her Di-
fiance from the fix'd Stars, or Appulre to them; her In-
grefs into the Ecliptic, or the Points of her Orbit where
that cuts the Ecliptic; and the Conjunaion, Diaance and
Eclipfes of Jupiter's Satellites. Of each of thefe in their
Order.
i. The firfi Method, by the Eclipfes of the Moon, is
very eafy and fufficiently accurate, were there but Eclip-
fes every Night. At the moment wherein we fee the
beginning or middle of a lunar Eclipfe by a Telefcope,
we have nothing to do but take the Altitude or Azimuth
of fome fixed Star, from which the Hour and Minute
are eafily found; or without the Altitude, if the Star be
in the Meridian. This Hour and Minute therefore, thus
found, and compared with that exprefs'd in the Tables,
give the Longitude.
2. The Moon's Place in the Zodiac is a Pbhnomenon
more frequent than that of her Eclipfes, but then the Ob-
fervation thereof is difficult, the Calculus intricate and
perplex'd, by reafon of two Parallaxes 3 fo that it's fcarce
practicable to any tolerable degree of Accuracy. Indeed
by waiting till the Moon comes into the Meridian of the
Place, and then taking the Altitude of fome remarkable
Star (the Latitude being fuppofed to be firfm known)
from this Altitude and the Latitude, we fhall be able to
find the Time pretty accurately, tho 'twill be better to
do it by fome Star in the Meridian. Now the Time be-
ing found, 'twill be eafy to find what Point of the Eclip-
tic is then in the Meridian or Mid-haven. Thus we fhall
have the Moon's Place in the Zodiac correfponding to the
Time of our Place. Then in the Ephemeris we find what
Hour it is in the Meridian of the Ephemeris, when the
Moon is in that part of the Zodiac: Thus we lhall have
the Hour and Minute of the two Places for the fame
Time; the Difference of which will give the Difference
of Longitude.
3. In regard there are many times when the Moon
cannot be obferv'd in the Meridian, there is therefore
another flill more frequent Phenomenon from which the
Longitude is fought; viz. the Moon's Appulfe and Recefs
from the fixed Stars: for from thence the Moon's true
Place may be inveffigated for the given Time of Obfer-
vation. But this Method, by reafon of the Parallaxes,
and the Solution of oblique fpherical Triangles, and the
various Cafes, is fo very difficult and perplex'd, that the
Mariners are not able to make ufe of it; nor is it necef-
fary to trouble the Reader with the Praxis thereof.
Thofe however who are difpofed to ufe it, will find very
confiderable Help from a fiarry Zodiac, publifh'd under
the Direaion of Dr. Halley, containing all the Stars to
which the Moon's Appulfe can be obferv'd.
4. To find the Longitude by the Moon's Ingrefs into the
Ecliptic; obferve the Moment of that Ingrefs: Then in
the Ephemeris, fee what Hour it is in the Meridian of the
Ephemeris, when that Ingrefs happens. The Difference
between thefe Times, gives the Difference of Longitude.
5. The Phenomena of Jupiter's Satellites are generally
preferred to thofe of the Moon, for finding the Longi-
tude; by reafon the former are lefs liable to Parallaxes,
and do, further, afford a very commodious Obfervation
in every Situation of that Planet above the Horizon.
Their Motion is very fwift, and muff be calculated for
every Hour, and for that reafon are not found in the corn.
mon Ephemerides, but are had elfewhere. Now to find
the Longitude by means of thefe Satellites, with a good
Telefcope obferve a Conjunalion of two of them or of
one of them with Japiter, or any other ,the like Appea-
L O
rance, and at the fame time find the Hour and Minut.
from the Meridian Altitude of fome Star; then confulting
Tables of the Satellites, obferve the Hour and Minute
wherein fuch Appearance happens in the Meridian of the
Place to which the Tables are calculated. The Diference
of Time, as before, will give the Longitude.
6. All Methods that depend on the Phs'nomena of the
Heavens having this one Defed, that they are not to be
obferved at all times j and being, befides, very difficult
of Application at Sea, by reafon of the Motion of the
Ship; there are fome, who, leaving the Moon and the
Satellites, have recourfe to Clocks and other Automata :
which, could they be made perfeffly jufl and regular,
fo as to move with the Sun without either gaining or lo-
fing, and without being affeded with the Change of Air
and of Climates; the Longitude would be had with all the
Eafe and Accuracy imaginable, nothing more being re-
quired but to fet the Machine by the Sun at the time of
Departure; and when the Longitude of any Place is dq.
fired, to find the Hour and Minute from the Heavens,
(which is done at Night by the Stars, and in the Day by
the Sun) for the Difference between the Time, thus ob-
ferved, and that of the Machine, gives the Longitude:
But no fuch Machine has been yet difcover'd. Where-
fore Recourfe has been flill further had to other Me-
thods.
7. Mr. WbiJlon and Mr. Ditton have propofed a Method
of determining the Longitude by the Flaih and Report of
great Guns. Sounds, 'tis known, move pretty equably
in all their Stages, whatever the fonorous Body be that
occafions it, or whatever the Medium that conveys it. If
then a Mortar or great Gun be exploded at a Place, whofe
Longitudc is known, the Difference between the Time
wherein the 4Flalh (which moves, as it were, inflanta-
neoufly) is feen, and the Sound, which moves at the rate
of four Seconds in a Mile, is heard, will give the Diflance
of thofe Places from each other; whence, if their Latitudes
be known, the Difference of Longitude will be likewife
known. Again, if the Hour and Minute of the Explofion be
known, (for the Place where it is made) by obferving the
Hour and Minute from the Sun or Stars, at the Place
whofe Longitude is required, the Difference between thofe
Times will give the Difference of Longitude. Again, if
the faid Mortar be loaden with an Iron Shell full of com-
buflible Matter, and pofited perpendicularly, it will carry
the fame a Mile high, which will be feen near a hun-
dred Miles; if therefore neither the Sound fhould be
heard, nor the Flafh feen, the Diflance of any remote
Place from the Place of the Mortar may be determin'd
from the Altitude of the Shell above the Horizon of the
Place unknown: and the Diflance and Latitudes known,
the Longitude is eafily found. According to this Scheme
stwas propofed to have fuch Mortars fix'd at proper Di-
flances, and at known Stations, on all the frequented
Coafis, Iflands, Capes, ec. and to be exploded at certain
Hours for the Obfervation of Marilers. This Method,
tho good in the Theory, yet is found ufelefs in the Prac-
tice; as being extremely troublefome, and yet preca.-
rious. It fuppofes that Sounds may be heard 40, 50, or
6o Miles; of which, 'tis true, we have Inflances,, but
they are very rare; and ordinarily the Report of a Can-
non is not heard above half fo far; and fometimes much
lefs. It fuppofes, again, Sound to move always with
equal Velocity; whereas, in fad, its Velocity is increafed
or diminished as it moves with or againfi the Wind. It
fuppofes, again, the Strength of Powder uniform; and
that the fame Quantity carries the fame Range; the con-
trary whereof is known to every Gunner. We fay no-
thing of thick cloudy Nights, when no Lights can be
feen; nor of flormy Nights, when no Sound can be
heard ; even at inconfiderable Difiances.
8. We have another Method of finding the Longitude,
propofed by the fame ingenious Gentleman Mr. Whifton,
viz. by the Inclinatory or Dipping-Needle. See Dipping-
Needle.
Longitude of the Earth, is its Extent from Wefi to Eaft,
according to the Situation of the .Equator; as the Lati-
tude of the Earth is its Extent in Surface from one Pole
to the other.
Longitude in the Heavens, is an Arc of the Ecliptic,
counted from the beginning of Aries, to the Place where
a Star's Circle of Longitude croffes the Ecliptic: fo that
it is much the fame as the Star's Place in the Ecliptic,
reckoned from   the beginning of Aries; which to find, fee
Place of the Sun or Star. Longitude of the Sun or Star
from tle next equinodial Point, is the Number of De-
grees and Minutes they are from the beginning of Aries or
Libra, either before or after them X which can never be
more than 8.o Degrees. Longitude, in Navigation, is alf*
the Diflance of a Ship or Place., EAs or Weft, from  anr
other.
LON


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