MECHANICS AND USEFUL ARTS. 77 



The design of these springs is to prevent the great waste of power incurred 

 in working the common fire-engine, by causing the descending arm of the 

 working lever (instead of being arrested by some solid part of the machine) to 

 give over all its momentum to springs of such strength as easily to offer the 

 requisite resistance to the blow, and of such elasticity as to give over nearly 

 all of then* power to the return-stroke of the engine. In the form commonly 

 seen, every successive stroke of the fire-engine starts from a dead rest, and 

 the power with which the stroke ends is totally lost in giving a blow to the 

 frame of the engine, giving it a very serious concussion, and causing painful 

 shocks to the arms of the firemen. The springs now proposed prevent all jar 

 and concussion from the stroke of the levers, and (supposing the springs to be 

 perfect, that is, to re-act without any loss of power from friction) they convert 

 the entire power with which the stroke is ended, into power acting in the 

 opposite direction for beginning the next stroke. They may be made of any 

 shape and any material so that they will arrest the whole momentum of each 

 stroke, and re-act with energy in starting the succeeding stroke. 



ZINC ROOFS. 



A square of 100 feet of zinc, at 22 ounces to the foot (No. 14 gauge), 

 weighs, when laid on with laps and rolls, 150 pounds; the same surface in 

 English slates, weighs 830 pounds, and in plain tiles 1.900 pounds. Zinc is, 

 consequently 5^- times lighter than slates, and nearly 14 times lighter than 

 tiles, in addition to the saving of surface and weight, by the flatness and 

 lightness of the framework and supporting walls. As compared with lead, 

 the density of zinc is 719, while that of lead is 11-35 ; the tenacity of zinc is 

 109'8, and that of lead is 27% from whence it follows that while a sheet of 

 zinc of equal thickness to one of lead is only two thirds the weight, its 

 strength or sustaining power is four times that of lead, while the cost is pro- 

 portionately low as applied to each material. 



IMPROVED CAST-IKON PAVEMENT. 



A new form of cast-iron pavement has recently been laid down in Phila- 

 delphia for experimental purposes. It consists of cast-iron plates, 12 feet 

 long, 3 feet 4 inches wide, and f inch thick ; 12 feet being the width of the 

 pavement to the curb. These plates are roughened on the surface by grooves, 

 * li inch apart, crossing each other at an oblique angle, so as to divide the sur- 

 face into diamonds. A cast-iron half-inch plate, with its two edges turned at 

 a right angle, so as to make flanges at the top and bottom, forming a girder 

 11 niches deep, is bolted to the columns of the building, making a support on 

 which the inner ends of the plates rest. The curb is of cast-iron, % inch thick, 

 11 inches deep, having a flange each side, at the bottom, and on the inside 

 only at the top ; it is made to slope slightly outward from the top to the bgt- 

 tom. This curb rests upon a brick wall, forming the outside wall of the cel- 

 lar, a good cement being interposed to make a water-tight joint ; the pieces 

 of curb have butt joints secured by a cast plate behind, riveted securely to 



