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A new Method of Surveying
Countries, or large Tracts
HE common Method of perforining
this Problem, is by saking and pratracting inaccessibleDiftances, which has been treated of ( in general) already'; and therefore, surposing that the Ingenious Surveyor cannot be incapable of applying it to this particular, I hall wholly omit this, and give you a few Method, invented by thofa Learned Mathematicians Mr Whifton and Mr Ditton, in order to whicb, let us premis. the following Lemmata.
1. All sounds are propagated almoft evenly, and are observed to move 8 measured Miles in 37 Seconds.
This is well known froin the last and most accurate Observations about we the Velocity of Sounds, which *Philo. Tran. are those of Mr * Derham.
II. An ordinary Mortar is easily able to cast a Projectile about a Mile in Perpendi. cular Heigth; and the Sound thereof may be heard near 20 Miles.
The truth of both thefe Propofitions have been sufficiently proved by divers Experiments made by Mr Whiston for that purpose.
III. A Projectile may be filld with Combustible Matters, to take Fire as foon as discharg'd, and continue burning till it comes to the Ground.
This all that deal in Rockets, Bombs, and Mortars, do very well know, and is found to be true upon Tryal.
IV. Fire, or Light about ia Mile high! will be visible in the Night time, when the Air is tolerably clear about, 70 Miles. )
This also hath been made inanifest by many Experiments made on purpose.
The Solution of the Problem.
Let a Shell that will take Fire as foon as discharg'd, and continue Burning will is comes to the Ground, be Thot perpendicularly about a Milé high out of a Mortar at any convenient place in a clear Night and this Discharge will by the Bearing, and Interval of the Flash and Sound, give the Diftance and Bearing of an Observer within the Hearing and Sight thereof, according to the foregoing Lemmata.
Example, Fig. 2.. Let the Bearing of a Shell disa charg'd out of a Mortar at C, and also the Interval of the Flath and Sound thereof be obferv'd by Persons fent to A. B. E. F. G.
H.I.D. places within 20 Miles round C, who inuft be furnished with an Instrument to measure Angles, for taking the Bearing of the shell, and a Thread 39.2 Inches long, with a Plummet faften’d to one end, which being suspended by the other end to a Pin, or Nail, and made to swing, will vibrate seconds by which the Interval between the Flash and Sound
may be nicely measured, and let their Olfervations be as follows.
To protract which,
i through the point C, draw the Meridian N. S. and lay
off the Angles NCD. 3*, NCA. 30°,NCG. 87° from the North towards the West. The AnglesNCE.40°,NCF 90"; from the North towards the East.The Angles SCI. 6°,SCH 500, from the South towards the West. Lastly, Lay off the Angle SCB 60" from the South towards the East. Then set off their diflances from the Mortar at C. viz. DC= 14 m. A C = 16 m. &c, according to the Observations. So will you have an exact Map of the place A.B.C.D.E.F.G.H.L, and the Scituation of the Villages about each Station may be easily taken by the Observer thereof after the common inethod of taking inacceflible Distances,
CORROL AR T's.
1. If each Observer were to let off a Rocket at his own Station, and take the mutual Bearing, of each others it wou'd be a great Check to any Error that could pollibly happen.
II. If a great Gun were discharg'd near the Mortar, and the Sound thereof used inft ad of that of the Mortar; it would be beard inuch further, and consequently a much larger Tract of Land might be surveyed at
A ready way to find a true Meridian line by the
The right Ascension of the Pole Star for this Year (1716) is 37 Minutes of Time; and it increales one Minute 16 Secondsevery ten Years: Therefore having at any time this Star's Right Afcenfion, and the Right. Afcenfion of the Sun both in Time, if you fubftra& the latter from the former, adding 24. Hours to the Right Ascension of the Pole-Star, when it is less than the Sun's the Remainder will be the time of the Star's, coming to the Meridian; at which time hang up two Pendulums between your Eye and the Pole-Sfar, and a Right-line drawn through them, will be a true Meridianline.
In the Survey just now treated of, you may use the fights of a good Circumferentor, or any other Surveying Instrument, ha: ving one of the lights long enough to take in the Pole-Star - If you wou'd kn: w, what Angle the Pole-Star inakes with the true Meridian at orher times, the following Table will thew.