NATURAL PHILOSOPHY. 211 



of the iron employed, testing the entire cable to a fixed proof strain, 

 and finally, by blows or impacts, as specially adapted for the discov- 

 t TV of false welds. The apparent increase of strength of bars re- 

 peatedly broken, first exhibited in the experiments of Mr. Lloyd, is 

 shown to be dne to increase of hardness, or of the di'lictilty of the 

 gliding to and fro of the particles so that whilst the resistance to 

 purely passive loads is increased, the resistance to impulsive forces is 

 enormously diminished at each fracture. The value of the government 

 hydraulic test of 11'4G tons per square inch is discussed, and the per- 

 manent set under this strain is stated to be -^ to -J-g- of the length. 

 While believing that a single application of this test docs not 

 materially injure the cable if good, Mr. Paget deprecates any attempt 

 to make the test more severe. 



PHOTOGRAPHY APPLIED TO TOPOGRAPHY. 



Photography has recently been applied hi France for the purpose of 

 topography with great success. By taking photographic views of any 

 prominent object from different points, the distance between these 

 points being measured, it is found possible to measure distances and 

 heights on the photographic views almost as accurately as by triangn- 

 lation. By this new process, by means of 20 views taken from 18 

 different points in less than GO hours, an accurate plan of the City of 

 Grenoble and of its environs, embracing an extent of more than 20 

 kilometers square (12i miles square), was executed in 60 days, which 

 by triangulation would have taken months. 



