Page:Cyclopaedia, Chambers - Volume 2.djvu/116

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Difference of thofe Times gives that of the Longitude of thofe Places. Now in the Ephemerides, we have the Mo- tions of the Planets, and the Times of all the Celeftial Phenomena, as the Beginning and Ending of Eclipfes, Conjunctions of the Moon with other Planets, its En- trance into the Ecliptic, iic. accurately calculated for fome one Place. Therefore if the Hour and Minute be known wherein any of the lame 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 confift in the exact finding of the Time, which is ea- fily had from the Sun's Altitude or Azimuth ; but the Defect lies in the Paucity of proper Appearances capable of being thus obferv'd : For all flow Motions (y.g. that of Saturn') are at once excluded ; as {hewing but little Difference in a confiderable Space of Time ; and it being here required that the Tbznommen be fenfibly varied in two Minutes Time, an Error of two Minutes in Time pro- ducing another of 33 Miles in the Longitude. Now there are no Phenomena in the Heavens that'have thefe Requi- fites, excepting the feveral Stages of an Eclipfe of the Moon ; her Longitude, or Place in the Zodiac ; her Di- flance from the fix'd Stars, or Appulfe to them ; her In- grefs into rhe Ecliptic, or the Points of her Orbit where that cuts the Ecliptic ; and the Conjunction, Diltance and Eclipfes of 3 up iter's Satellites. Of each of thefe in their Order.

1. The firft 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 Phenomenon more frequent than that of her Eclipfes, but then the Ob- fervation thereof is difficult, the Calculus intricate and perplex'd, byreafon of two Parallaxes; fo that it'sfcarce 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 firft known) from this Altitude and the Latitude, we ihall 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 mall have the Hour and Minute of the two Places for the fame Time 5 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 ftill 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 inveftigated 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.

f Thofe however who are difpofed to ufe it, will find very

confiderable Help from a ftarry Zodiac, publifti'd under the Direction 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 3upiter'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 muft be calculated for every Hour, and for that reafon arc not found in the com- mon Ephemerides, but are had elfewhere. Now to find the Longitude by means of thefe Satellites, with a good Telefcope obferve a Conjunction of two of them or of one of them with 3uj>iter, or any other the like Appea-

rance and at the fame time find the Hour and Minute from the Meridian Altitude of fome Star ; then conlultin. 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 Difference of lime, as before, will give the Longitude.

6. All Methods that depend on the Phenomena of the Heavens having this one Defeft, that they are not to be obferved at all times ; 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 perfeaiy jufl and regular fo as to move with the Sun without either gaining or lo^ fing, and without being affefted 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 de- 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 Lontitude : But no fuch Machine has been yet difcover'd. Where- fore Recourfe has been ftill further had to other Me- thods.

7. Mr. Whifion and Mr. Ditton have propofed a Method of determining the Longitude by the Fla/h 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 Lo„giu,Jc is known, the Difference between the' Time wherein rhe iFlafh (which moves, as it were, inftanta- neoufly) is feen, and the Sound, which moves at the rate of four Seconds in a Mile, is heard, will give the Diltance 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 co'm- buftible 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 determih'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 'twas propofed to have fuch Mortars fix'd at proper Di- ftances, and at known Stations, on all the frequented Coafts, Iflands, Capes, i$c. and to be exploded at certain Hours for the Obfervation of Mariners. 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 60 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 fafl, its Velocity is increafed or diminifhed as it moves with or againft 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 Diftances.

8. We have another Method of finding the Lontitude, propofed by the fame ingenious Gentleman Mr. Whifion, ■viz. by the Inclinatory or Dipping-Needle. See Vipping- Needle.

Longitude of the Earth, is its Extent from Weft 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 the next equinoctial Point, is the Number of De- grees and Minutes they are from the beginning of Aries or Libra, either before or after them ; which can never be more than 18.0 Degrees. Longitude, in Navigation, is alfo the Diflance of a Ship or Place, Eaft or Weft, from an- other.

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