MECHANICS AND USEFUL ARTS. 77 



spots become bright, whilst the sunken portions arc dull, and the casting 

 acquires a dark, gray appearance, with an antique lustre. "\Yithout biting, the 

 color is light-gray. Some casts of medals taken with this alloy in plaster of 

 Paris were so successful, that the finest contours and the legend, which in the 

 original was only legible with the lens, were completely reproduced. Calcu- 

 lated for 100 parts, this alloy consists of 27'27 bismuth, 59 '00 lead, and 13'61 

 tin. As bismuth is expensive in comparison both with lead and tin, the 

 quantity of lead might be increased, and that of the bismuth diminished, 

 without injury to the valuable properties of the alloy. It is probable this 

 mixture may be adapted for typographical purposes. Polytcchn. Cent mill., 

 1S-37, p. SS8. 



NEW MATERIAL FOR MOULDS, ETC. 



It is proposed to introduce a vast improvement in the casting of metals, by 

 substituting compressed carbon for the sand or clay usually employed. The 

 advantage to be derived is, that the same mould may be used over and over 

 again without injuring the smooth surface of the cast material. The carbon 

 to be employed, which is manufactured under a patent recently granted to 

 Mr. Buhring, of England, is comparatively pare, and can be moulded into 

 any shape and form required. The same material has been sucessfully ap- 

 plied to the manufacture of crucibles, and these crucibles are by many con- 

 sidered superior to any others. Another purpose to which the compressed 

 carbon is applicable is the manufacture of battery plates; and it is antici- 

 pated that electric telegraph companies would effect a vast saving in the cost 

 of their batteries, by employing carbon in connection with iron, instead of 

 the zinc and copper plates now used. 



ON THE SHAPE OF BRICKS. 



Mr. George Gilbert Scott, an eminent English architect, in a recently pub- 

 lished work, makes the following observations on the form or shape of 

 bricks. He says : " The shape of a brick has a great influence on the effect 

 in work. Our bricks are too short for their thickness they should either 

 be thinner or longer. I should say thinner for small buildings, and longer 

 for large ones. If, for instance, we had for large buildings facing-bricks of 

 the usual thickness, but nearly a foot long, they would look well, and would 

 work in with a backing of common bricks, if necessary; but for small build- 

 ings, bricks of the usual length and breadth, but only two and a half inches 

 in thickness, would look best. In the north of German}', bricks were used in 

 the middle ages, for large buildings, of much greater size than we now use 

 them; this would have been good had the thickness been kept moderate; 

 but that being increased in proportion, the bricks Avere often insufficiently 

 burnt; and, except in buildings of gigantic size, they looked clumsy. The 

 Roman brick, which was twice the length of ours, and little more than half 

 its thickness, Avas in the other extreme but it is the better side to err on. 

 Their length ensures good bonding, AA'hile their thinness causes them to be 

 thoroughly burnt." 



DUTY OF STEAM ENGINES. 



The folloAving interesting practical examples of the difference between the 

 actual and theoretic duty in different descriptions of steam-engine , is ex- 



7* 



