CHAP. V.] 



TELEGRAPHIC LINES. 



623 



place, which is done by means of a button on the outside terminating 

 the lever, seen on the upper side of the box. 



Electric alarums in mines have received an application of great 

 interest with regard to the lives of the miners. The simple presence 

 of fire damp when its proportion in the air of a mine is great enough 

 to be dangerous, may be indicated automatically by the use of an 

 apparatus in electric communication with a battery and an alarum. 

 The principle on which this apparatus, invented by M. Ansell, is based, 

 is this. 



We know that if two gases of unequal density be separated by a 



FIG. 407. Breguet's vibrating alarum. 



FIG. 408. Aubine's A ibrating alarum, with catch. 



porous membrane, each of them will cross the membrane with a 

 velocity peculiar to it. At the end of a certain time, there will be a 

 mixture, but as the less dense gas crosses the porous partition in 

 greater abundance than the other, it results that in the space occupied 

 by the latter there will be an alteration of pressure. We will see 

 Low this phenomenon is made use of in the fire-damp indicator 

 (Fig. 409). 



A curved tube has one of its branches terminated in a funnel or in 

 the form of a vessel closed by a plate of porous material m. The tube 

 contains mercury whose level is the same for each branch under ordi- 



