VOL. LXXVII.] PHILOSOPHICAL TRANSACTIONS. 243 



taining a fine intersection of the signal of the Boulemberg, a hill of some note 

 behind the town of Boulogne, and one of the stations used by the French 

 Academicians in the execution of their triangles. The advantages of obtaining 

 a triangle of this magnitude, whose sides are respectively in length about 45, 

 36, and 25 miles, are too obvious to require any comment. The high chalk 

 cliffs near Folkstone prevent Dover Castle from being seen from Lid, or any 

 where in the plain of Romney-marsh. Hence it will become necessary to form 

 2 small triangles to the northward of Tatterlees, in order to obtain an intersec- 

 tion of one of the turrets of the keep of that castle. Of the centre of the 

 keep, it will be perceived, by the strong dotted lines, that the French Acade- 

 micians have procured, from their stations at Calais, Blancnez, and Audinghen, 

 an acute intersection at Dover Tower, making in the whole an angle of 

 28° 16' 20". 



The points which are obviously the best for connecting our triangles with 

 those of our neighbours, are the Boulemberg, Blancnez, and Calais, provided 

 we could by any means obtain as good an intersection of the last as we are cer- 

 tain of getting of the 2 first ; but the breadth of the range of chalk hills being 

 little different on our side from what it is on theirs, by confining ourselv^es to 

 such a base as they will afford us, we cannot any way obtain an intersected 

 angle at Calais greater than about 29° or 30°. 



Thinking that possibly from St. Peter's church in the isle of Thanet the 

 tower of Notre Dame at Calais may be seen, I have extended dotted triangles 

 into that part of Kent ; because, if the united heights should not be sufficient 

 to raise the top of the tower above the curvature of the sea, which is the only 

 thing to be doubted, we are always certain, that the signal of Blancnez may, 

 by means of the Indian lights, be easily seen, since the whole range of chalk 

 hills behind Calais are discovered with the naked eye from the isle of Thanet, 

 when the weather is tolerably clear. 



Having in this manner ascertained the relative situations, with regard to the 

 coast of England, of 3 points on the coast of France, forming a triangle whose 

 sides and angles we already know from their trigonometrical operations, we shall 

 in like manner be enabled to determine the situation of the point called m near 

 Dunkirk, where the meridian of the Royal Observatory of Paris intersects a line 

 drawn from the great tower of Dunkirk to that of n. d. at Calais. The distance 

 M. p. on the meridian of Paris, between that point m and the parallel of Green- 

 wich, will then be had ; and that being added to 13341/ fathoms, the distance 

 northward from the Paris Observatory, we shall have the total terrestrial arc, com- 

 prehended between the parallels of the 2 Observatories, answering to an arc in 

 the heavens of 2° 38' iQ", or a difference of latitude between 51'' 28' 40" and 

 48° 50' 14". In like manner the distance from Greenwich, on the parallel of 



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