646 PHILOSOPHICAL TRANSACTIONS. [anNO 1755. 



the earth the radius. For expedition thus ; draw two lines, making an angle of 

 17° 56': on one of them set ofF the perpendicular from the comet's projected 

 place, and raise a perpendicular to the other ; or, which is the same, from the 

 comet's real place in the parabola ; and let fall a perpendicular, that is the tan- 

 gent of the geocentric latitude. 



One observation of a known comet will, on such a scheme, determine in some 

 measure its whole course; for, from the earth's place, draw the observed longi- 

 tude of the comet, where that cuts the projection of the parabola is the comet's 

 place ; to which if the observed latitude agrees, it confirms it : then the other 

 data being already known, and one place given, its whole course may be traced. 

 Such a scheme may be also of use to find the periods of comets, where the de- 

 scription of one is not good enough to find its orbit by ; for if an old comet was 

 seen in August, in ^XX", or in 25, with south latitude, or very bright in January, 

 it cannot be the comet of l682 ; but if in November in y , near the ecliptic, it 

 may. It then remains to see, whether the rest of the description will agree 

 with the course it would in that case take ; if it does, then, as the account is 

 more or less perfect, there is a greater or less probability of its being the same. 



r 



A Table showing where the Comet may be expected to begin to appear 



any Month. 



January . . 

 February. . 

 March . . . . 



April , 

 May . 

 June . 

 July. 



. . end 

 begin 



end 

 begin 



end 

 begin 



end 

 begin 



end 

 begin 



end 



August . . 

 September . 

 October . 



Novem. . . 

 Decern. . . 



begin 

 . mid. 



end 

 begin 



end 



Scarcely to be »een 



Retr. between 30° and 15" f 



30 and 1 5 W 



30 and « 



15 and OK 



Stat. 10 r and 20 K 



.... middle T 



Dir. begin. Q 



begin. Q 



end 



.... begin, n 



.... middle n 



.... end n 



Stat. 25 and 30 n 



Retr. end n 



begin, n 



5 n and 20 O . . 



.... begin. 



begin. end T . . 



.... begin. T 



Lat. 

 Small increasing S. . 



Small N. or S 



Small N. decreasing. 

 Small N. decreasing. 

 Small N 



N 



7 weeks after perihelion. 

 \ a month after perihelion. 



> 2 or 3 weeks after. 



about perihelion. 

 1, 2, or 3 weeks. 



■ N. increasing 2 to 5 weeks before. 



}■ 



Small increasing N. 

 Small S. or N 



Small S 



1 Small S. or N. 

 / very faint . 



5 to 8 weeks before. 

 2 months before perihel. 



2 or 3 months. 



3 months before perihel. 

 11 to 14 weeks. 



LVIII. An Extraordinary and Surprising Agitation of the tVaters, though * 

 without any perceptible Motion of the Earth, having been observed in various 

 Parts of this Island, both Maritime and Inland, on the same Day, and chiefly 

 about the Time that the more Ftolent Commotions of both Earth and IVaters 



* See the note on the letter from R. Philips. — Orig. 



