86 ANNUAL OF SCIENTIFIC DISCOVERY. 





crushed into powder by machinery, and afterwards heated to a 

 temperature of about 140 C. It then remains in the state of a 

 dry, fine powder, somewhat similar in its consistency to moulders' 

 sand, and in this form it is employed in the streets. The roads to 

 be paved are first covered with a layer of concrete made of gravel 

 and cement, and this layer is carefully dried before the applica- 

 tion of the asphalt cover. The asphalt powder is then reheated 

 and spread over the surface of the concrete in an even layer of 

 about 4 centimetres, or 1 3-5 .inches in thickness throughout. After 

 this the powder is rammed and compressed by means of heated 

 cast-iron rams worked by hand. This being done, a heated roller 

 is passed over the surface. The roller weighs about 400 weight, 

 and is repeatedly traversed over each short length of pavement 

 newly rammed in. Two larger rollers, one of 1,600 weight and 

 one of about 2 tons' weight, are afterward employed for flattening 

 down the surface of the whole. The pavement is finished and 

 ready for use immediately after cooling, say 2 or 3 hours after the 

 first roller has completed its work. The asphalt pavement has 

 now had an extensive and complete trial, and its advantages are 

 very numerous. There is neither dust nor mud produced by it, 

 and its surface wears no more than 1 millimetre, or one-twent} r - 

 fifth of an inch in thickness per annum in streets having a lively 

 traffic. At the beginning there is a compression caused by the 

 weight of the vehicles rolling over the pavement, but the whole 

 gets soon into a state of uniform density, and the street then re- 

 mains in a perfect state for a long time, requiring very little repair. 

 There is no noise whatever from the wheels of carriages in asphalt- 

 paved streets, so that there is a certain danger caused by this to 

 pedestrians from the want of warning of the approaching car- 

 riages. This, however, disappears by degrees, as the public be- 

 come more and more acquainted with this kind of pavement. The 

 tractive force required by the carriages passing over asphalted 

 streets is very considerably reduced, and still more important is 

 the reduction of the wear and tear of carriage-wheels, springs, and 

 axles, a reduction which is due to the absence of all concussion 

 and vibration in the rolling of the carriage-wheels over the smooth 

 and uniform surface of the street. Engineering. 



IMPROVED INSULATOR. 



An Englishman named Hooper is reported to have perfected 

 the application of caoutchouc for insulation of telegraphic wires, 

 so as to supersede gutta percha by an altogether superior article. 

 Tests for inductive resistance on a knot of the Ceylon cable core, 

 manufactured by Mr. Hooper, showed that the time of falling 

 from a tension of full charge to that of half charge was 300 min- 

 utes. The Atlantic cable falls in 60 or 70 minutes. The inductive 

 resistance was to that of gutta percha as 1 .36 to 1. Its permanency 

 of insulation at high temperatures is the most remarkable quality 

 of this coating. At 212 F. its insulation is more tenacious than that 

 of gutta percha at 100. It is manufactured at a temperature of 

 280. It is also nearly impermeable to moisture even under pres- 



