SIR HUMPHRY DAVY ? S SAFETY-LAMP. 



enter, forbade the white-hot gas within to escape without parting 

 with so much of its heat in the transit as to deprive it of the 

 character and properties of flame ; so that, although it passed 

 into the external explosive atmosphere, it was no longer in a 

 condition to inflame it. 



The lamp thus serves a double purpose : it is at once & protec- 

 tion and a warning. It protects, because the flame within cannot 

 ignite the gas outside the lantern. It warns, because the miner, 

 seeing the gas burning within the lantern, is informed that he is 

 enveloped by an explosive atmosphere, and takes measures 

 accordingly to ventilate the gallery, and meanwhile to prevent 

 unguarded lights from entering it. 



Nothing can be imagined more triumphantly successful than 

 this investigation of Sir Humphry Davy. Some philosophers 

 have the good fortune to arrive at great scientific discoveries in 

 the prosecution of those inquiries to which the course of their 

 labours leads them. Some are so happy as to make inventions of 

 high importance in the arts, when such applications are suggested 

 by the laws which govern the phenomena that have arisen in their 

 experimental researches. But we cannot remember any other 

 instance in which an object of research being proposed to an 

 experimental philosopher, foreign to his habitual inquiries, having 

 no associations with those trains of thought in which his mind has 

 been previously involved, he has prosecuted the inquiry so as to 

 arrive not only at the development of a natural law of the highest 

 order, the fruitful parent of innumerable consequences of great 

 general importance in physics, but at the same time, to realise an 

 invention of such immense utility as to form an epoch in the 

 history of art, and to become the means of saving countless 

 numbers of human lives. 



WHITWOETH'S 

 MICROMETRIC APPARATUS. 



AMONG the many admirable machines produced by Mr. Joseph 

 Whitworth at the Great Exhibition, was a micometric apparatus 

 for establishing uniform standards of magnitude for taps, axles, 

 and other important component parts of machines. By this 

 instrument, magnitudes, so minute as even to elude the micro- 

 scope, are submitted to mechanical measurement. 



Two perfectly plane and smooth metallic surfaces are first 

 190 



