624 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



nary circumstances, that is to say, when the air in the gallery is pure. 

 But if the hydro-carbon gas is disengaged near the apparatus, it 

 penetrates the porous plate and increases the pressure in this branch 

 of the tube and drives back the mercury in the other branch. The 

 mercury, in thus rising, brings the two electrodes, positive and negative 

 a, I, of a battery into contact, by means of a metal rod/. The current 

 passes, makes the alarum sound, or sends any kind of telegraphic 

 signal either to the inside or the outside of the mine. 



The same apparatus will indicate the presence of any gas that is 

 heavier than the air, as carbonic acid or hydrosulphuric acid. It is 



FIG. 409. M. Ansell's fire-clamp indicator. 



enough for this purpose to make contact in the part of the tube which 

 is situated below the porous plate. 



Ansell's fire-damp indicator has been tried with success in several 

 mines in England and France. 



V. THE LIGHTNING CONDUCTORS. 



The superiority of the electric over the air telegraphs results 

 principally from the rapidity with which public or private messages 

 may be transmitted, whatever may be the distance within certain 

 limits between the extreme stations. A few seconds or a few minutes 

 at most, suffice for this docile agent to clear thousands of miles. Bat 

 this is not the only reason that led to the rejection, as superannuated, 

 of a mode of correspondence which appeared for more than forty 



