MECHANICS AXD USEFUL ARTS. 5o 



It is evident that, by varying the proportions of steel and cop- 

 per in the compound wire, any desired relative strength can be 

 given between the limits of 1.72 and 7.47 ; and at the same time 

 any desired conductivity can be had alono^ with it. 



It will be seen, however, that a high relative strength is more 

 costly than a low one ; for the reason that steel possesses a less 

 specific conductivity than copper, and this difference of conduc- 

 tivity is greater than the difl'erence of cost. 



But, in the construction of lines of telegraph, while an increased 

 relative strength adds to the cost of the wire used, it, on the other 

 hand, effects a saving in the number of poles and insulators re- 

 quired, thus reducing the total cost of material and its transporta- 

 tion, which is often of great importance ; therefore, increased 

 relative strength is on the whole more economical. 



We can get the conductivity of a No. 8 galvanized iron wire 

 by using a compound wire weighing only 80 pounds per mile. 

 Such a wire would be handled with the greatest ease, as a man 

 could readily carry a mile or more upon his back. 



All the advantages of a heavy iron wire, which would weigh 

 from 500 to 700 pounds per mile, can be secured by a compound 

 wire weighing less than 175 pounds per mile. 



Other sizes than these can be made possessing intermediate or 

 greater relative strength, and of any desirable conductivity. 



The foregoing results are based on the employment of a copper 

 which shall possess seventy-eight one-hundredths of the conductiv- 

 ity of a chemically pure copper wire. 



The standard unit of conductivity here employed is that of a 

 round copper wire one-twentieth of an inch in diameter, chemi- 

 cally pure, and 1 foot in length. 



ARTIFICIAL STONE. 



A method of manufacturing artificial granite has recently been 

 patented in England. The materials of this artificial granite are 

 disintegrated natural granite, mixed with clay, together with 

 pounded glass, lava, and iron slags. The disintegrated granite is 

 obtained by submitting fragments of natural granite to a strong 

 heat, about 700° or 800° C, in an oven, by which, after a suffi- 

 cient time, it becomes dissolved into a granitic sand, the constitu- 

 ent parts of which, quartz, or feldspar, possess great powers of 

 adhesion. One part of this granitic sand is then mixed with an 

 equal quantity of pounded glass, or the constituents of glass, or 

 lava, or iron slag, to which is added from 20 to 30 parts of refrac- 

 tory clay, or from 30 to 50 parts of ordinary clay. This mixture 

 is thoroughly kneaded together with a sufficient quantity of water 

 to make it of a pasty consistency. It is then moulded to any form 

 required, and submitted to a degree of heat sufficient to vitrify the 

 mass for about 36 hours, which converts it into a durable substance 

 resembling granite. The artificial granite thus produced may be 

 moulded into any forms required to render it suitable for various 

 kinds of buildings, fortifications, docks, and other engineering struc- 



