1871-] A New Mechanical Agent. 359 



waves of the same length. The waves, however, travelling 

 along any single line of propagation may be separated 

 from each other by long intervals. The greater or less 

 rapidity with which they succeed one another, will merely 

 give rise to a greater or less intensity of effect at the 

 recipient surface; a greater or less brightness in the case of 

 the luminous waves; a greater or less heating effect in the 

 case of the thermal waves; a greater or less photographic 

 effect in the case of the actinic waves ; a greater or less 

 amount of fluorescence in the case of those waves which 

 stimulate that property into action. In the case of lumi- 

 nous waves, they might be separated by intervals of 18,500 

 miles, and yet produce continuous vision ; but waves sepa- 

 rated by intervals of only a mile would produce a very 

 great amount of intensity. It is owing to this largeness 

 of the interval between the waves travelling along any 

 individual line of propagation, that rays from different 

 sources are found to cross each other in all directions 

 without interfering one with another. 



(To be continued). 



Cj^ j 



VII. A NEW MECHANICAL AGENT. 



A JET OF SAND. 



OW to cut or carve, mechanically, hard substances, 

 such as stone, glass, or hard metals, in an expeditious, 

 accurate, and economical manner, has always engaged 

 the attention of engineers. At the present time the rapidly 

 increasing cost of manual labour makes improvements in 

 this direction more needful. The discovery and utilisation 

 of opaque crystallised carbon, cheaper than transparent dia- 

 monds, but perhaps equally durable, has gone far in this 

 direction. Now, Mr. B. C. Tilghman, of Philadelphia, comes 

 forward, and shows that a jet of quartz sand thrown against 

 a block of solid corundum will bore a hole through it 

 1 j inches in diameter, ij inches deep, in 25 minutes, and this 

 with a velocity obtainable by the use of steam as a propelling 

 power, at a pressure of 300 pounds per square inch — a re- 

 markable result, when we consider that corundum is next to 

 and but little inferior to the diamond in hardness. 



At the meeting of the Franklin Institute, held February 

 15th, 1871, the resident secretary, Dr. W. H. Wahl, intro- 

 duced this invention, illustrating his description of it by 



