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constructed under the direction of M. Fresnel. These lenses were 

 3 feet in diameter, and were composed of numerous pieces; the light 

 they gave far exceeded that of any of our lighthouses, appearing at 

 the distance of 48 miles, like a star of the first magnitude. Staffs 

 were also erected near the lamps, but these were only occasionally 

 visible. 



These lamps were placed under the care of proper attendants on 

 stations selected upon Fairlight Down and near Folkstone Turnpike, 

 and the observers then crossed the Channel on the 24th of Septem- 

 ber 1821, and stationed themselves at Blancnez. Their observations 

 were for some time retarded by tempestuous weather, but were com- 

 pleted on the 7th of October. They then removed to Montlambert, 

 a fort situated on a height near Boulogne, where a further delay oc- 

 curred in consequence of an accident that had happened to the lamp 

 at Fairlight. On the 14th they returned through Calais to Fairlight. 

 Here they at length succeeded in discovering the wooden pipe by 

 which General Roy had marked his station, and which had been bu- 

 ried 4 feet deep in the earth. In order to preserve this point, a mill- 

 stone, having the words " Roy's Station " cut upon it, was placed 

 level with the surface of the ground, its centre being precisely over 

 the centre of the pipe. 



The party next proceeded to Folkstone ; and in order to carry on 

 the series towards London, stations were selected on Stede Hill and 

 Wrotham Hill, which were connected with Folkstone by an interme- 

 diate point on Tolsford Hill, from which the stations on the French 

 coast are visible, and the triangles connected with the church of 

 Notre Dame at Calais. It was found impossible to connect the tri- 

 angles directly with the base measured by General Roy upon Houns- 

 low-heath, on account of many intervening buildings since erected, 

 which now intercept the view of one end of the base from the other 

 end. The summer of 1822 was employed in the choice of stations, 

 one of which was the temporary meridian mark erected near Ching- 

 ford for the Royal Observatory, and which was fixed upon in order 

 that the side of one of the triangles might coincide with the meridian 

 of Greenwich. Observations were next made from Leith Hill, Wrot- 

 ham Hill, Stede Hill, Crowborough, and Severn-droog Castle, on 

 Shooter's Hill, which were connected with Greenwich by the north- 

 west pinnacle of Westminster Abbey and the cross of St. Paul's, and 

 afterwards by means of Hanger Hill, with Fairlight Down and Folk- 

 stone turnpike. A difficulty occurred in observing from the station 

 on Shooter's Hill the signal staff erected upon Hanger Hill, in con- 

 sequence of the intervening smoke of London. This was obviated 

 by a contrivance of Colonel Colby, consisting of several tin plates 

 nailed to the staff, and disposed one above another at such angles as 

 to reflect in succession the sun's rays in the direction of Shooter's 

 Hill, and thus to answer the purpose of a heliostat. Each plate gave 

 in its turn a neat image of the sun, resembling a fixed star, which 

 was seen through a smoke so thick as to render the hill itself in- 

 visible. 



