No. 244.] 191 



laid in mortar, the span of which is six hundred feet, and the height 

 of the arch 750 feet. The middle arch of the Westminster bridge, 

 London, is only 76 feet span, and springs from about two feet above 

 low water mark. The span of the middle arch of Black Friar bridge 

 is 100 feet, built in the elliptical form, and the most stupendous 

 bridge in all Europe is that over the Tave in Glamorganshire, con- 

 sisting of one arch, the segment of a circle, the diameter of which is 

 175 ; the chord of the segment is 140 feet ; and the height 35 feet. 

 You will, therefore, observe how far superior the Chinese arch is to 

 the best in enlightened Europe, by whom the Chinese are called a 

 benighted people. Our common mortar is composed of lime, sand 

 and water, mixed until it forms a paste, which dries and becomes 

 almost as hard as stone, and the strength of our walls is in proportion 

 to the excellence of the lime or calcareous earth from which this 

 substance is made, being usually combined with other matters; it is 

 good in a ratio equivalent to its detachment from these combinations, 

 which is effected by exposing the lime stones to a very strong heat, 

 in what is termed a lime kiln ; these stones consist of lime combined 

 with carbonic acid gas in a solid state, and before being burned is 

 termed carbonate of lime; when red heat is applied the gas immedi- 

 ately flies off, and the substance left is termed quick lime, and is 

 soluble in water, when water is applied with a view of slacking it, it 

 cracks and falls into fine powder, a portion of the water escapes in 

 the form of steam, but much of the largest quantity combines with 

 the lime and becomes solid, and forming a substance called by chem- 

 ists hydrate of lime, and by masons slacked lime. 



Limestones affording lime appropriate for mortar are of two kinds, 

 viz : those which consist of nearly pure carbonate of lime ; and 

 those which consist of carbonic acid and a portion of iron, magnesia 

 or clay. That made from the first is far superior, as it dries at once 

 when exposed to the air, and becomes very hard, and will not soften 

 in water applied when perfectly dry ; if, however, water is kept in 

 contact vnih it before perfectly dry, as in drains, &c., it will never 

 set or harden, and is therefore not used for hydraulic purposes. Lime- 

 stones containing clay, especially if they are ferruginous, are termed 

 when burned, hydraulic or water lime, and when mixed with sand sets 

 even under water. Builders are generally of opinion that the best 

 lime is obtained from the hardest limestone ; this is not so, however. 

 Lime made from porous, soft stone, absorbs from the atmosphere the 



