28 ANNUAL OF SCIENTIFIC DISCOVERY. 



angle, which practice enables the person performing this duty to ac- 

 complish at once, there are two parallel shadows thrown upon opposite 

 side-s of the inner surface, which, by another deflection, can be made to 

 come to a point at the lower end. The appearance which these 

 shadows assume determines the question whether the barrel is straight 

 or not, and if not, where it requires straightening. Although this 

 method is so easy and plain to the experienced workman, to the un- 

 initiated it is perfectly incomprehensible, the bore of the barrel 

 presenting to his eye only a succession of concentric rings, forming a 

 spectacle of dazzling brilliancy, and leaving the reflected line in as 

 profound a mystery after the observation as before. 



At present, the mirror is discarded, and the workman holds the 

 barrel up directly to the pane of glass, which is furnished with a 

 transparent slate, having two parallel lines drawn across it. The only 

 purpose subserved by the mirror was that of rendering the operation 

 of holding the barrel less tiresome, it being easier to keep the end of 

 the musket presented to the line pointing downwards than upwards. 

 Formerly this means of detecting the faults, or want of straightness in 

 the barrel, was the secret of one man, and he would impart it to no 

 one for love or money. He was watched with the most intense 

 interest, but no clue could be obtained to his secret. They gazed into 

 the barrel for hours, but what he saw they could not see. Finally, 

 some fortunate individual stumbled upon the wonderful secret, dis- 

 covered the marvellous lines, and ever since it has been common 

 property in the shop. Atlantic Monthly. 



SMITH'S AIR-LIGHT. 



A highly successful and economical application of the oxy-hy- 

 drogen lime light has recently been made by Dr. G. H. Smith, of 

 Rochester, New York ; atmospheric air being substituted in the place 

 of oxygen gas, and carburetted hydrogen, or common gas, in the place 

 of hydrogen. The rendering of these substitutes applicable for the 

 production of the oxy-hydrogen light a project which at first seems 

 chimerical is explained by the inventor as follows: "Common air 

 contains one part of oxygen and about three parts of nitrogen gas ; 

 hence it would require four parts of air to give the amount of oxygen 

 required for illuminating purposes ; i. e. to obtain one part of oxygen 

 four feet of air would be needed, as three feet would be nitrogen. 

 Now, the great difficulty in using air as a substitute for pure oxygen, 

 in the production of the lime light, is due to the fact (and to this 

 solely) that when four parts of air are combined with one of common 

 gas, the gas is so greatly diluted as to prevent its burning readily, and, 

 what is still worse, if combustion was complete, the nitrogen, not being 

 combustible, would fly off unconsumed, and carry away the heat gen- 

 erated to such a decree as to render the luminosity of the cylinder of 



^J V / 



lime (against which the ignited jets of the combined gases are direct- 

 ed) of no practical value. But if an amount of heat, from any source, 

 is applied to the current of air previous to ignition, sufficient to supply 

 the loss of heat abstracted by the inert nitrogen, at the time of com- 

 bustion, no heat is lost upon the lime, and the whole power of the oxy- 

 gen is obtained as though no nitrogen was present. Hence, by sup- 

 plying a current of preheated air to one of common gas ignited, an 



