LIGHT HOUSES. 



above tho sterlings, is 450 feet, scarce half the width of the rive r ; 

 inid b!;>w the sterlings, the water-way is reduced to 194 feet. 

 Thu a river SOO feet wide, is here forced through a channel of 

 194 feet. This bridge, it is expected, will be taken down ere 

 Ion", and ono of cast iron has been proposed to be erected in lieu 

 of it. Opinions are much divided, however, as to the propriety of 

 such an erection , but, as to the necessity of anew bridge there 

 can be no doujbt ; the piling of the old one being iu such a con- 

 dition as to place the whole structure in a veryprecarious state. 



Of modern bridges tin-re are few, if any, which excel the West- 

 minster and Blackfnars' bridges over the river Thames. The for- 

 mer is 1220 feet long, and 44 feet wide, having a commodious 

 foot-path on each side for passengers. -It consists of IS arches ; 

 was finished in 1750, and cost 389,500/. The latter, nearly oppo. 

 site the centre of the city of London, was finished in 1770 : it con- 

 sists of 9 large and o!".iant arches, nearly eliptical, of which the 

 centre arch is 100 feet wide : the breadth of the bridge is 42 f.et, 

 and its length from wharf to wharf 995. It cost 150,840/. 



[Hut ton. Perronet. Pantolog. 



SECTION II. 



Light- houses, exemplified by those of Phasclus, Pharos, and 

 the Eddystone Rocks. 



A LIGHTHOUSE is a building erected upon a cape or promontory 

 on the sea-coist, or upon some rock in the sea, and having on its 

 top in the night-time a great lire, or light formed by candles} which 

 is constantly attended by some careful person, so as to be seen at 

 a great distance from the Kind. It is used to direct the shipping on 

 the coast, that night otherwise ran ashore, or steer an improper 

 course when the darkness of the night and the uncertainty of cur- 

 rents, &c. might render thrir situation with regard to the shore 

 extremely doubtful. Lamp-lights are, on many accounts, prciVr. 

 able to cial-fires or candles ; and the effect of these nm !> i:u 

 crea-ed by placing them either behind glass-hemispheres, orbefbr; 

 properly disposed glass or metal reflectors, which last method is 

 now very generally adopted. 



In the supplement to the Encyclopedia Britannica, under the 

 word Reflector, it is stated that " Mr. Thomas Smith, tin-plate 

 worker, Edinburgh^ seems totiave conceived the idea ofillumin.it- 



