398 FIRE-DAMP INDICATOR 



action proceeds with the diffusion, honco wo never obtain all the effect calculated 

 upon; but directly diffusion ceases, effusion going on empties the chamber of gas, so 

 that if the instrument be held long enough in the same atmosphere, the hand will 

 return to zero, whence it started, and remain there till the instrument is taken into a 

 purer atmosphere, or into one more fully charged. If into an atmosphere more fully 

 charged, the barometer will rise, and this increased reading must bo added to the 

 other reading, so as to obtain the amount present at the new place. Thus in one 

 reading it gives 3 per cent, of fire-damp ; this atmosphere remains, and the hand 

 returns to zero, but the instrument is placed in an atmosphere which gives 7 per cent, 

 on its face, therefore the true reading is 7 + 3 = 10 per cent., which can be confirmed 

 by taking the instrument into the intake gallery for a few minutes, and then putting 

 on the cap ; carry back the instrument to the suspected atmosphere, and it will at once 

 indicate the 10 per cent. 



Effusion has been a source of great trouble in the elimination of a suitable substance 

 for a diffusion septum, but Mr. Ansell has just now happily found that Sicilian marble 

 supplies his wants, for this substance permits diffusion to take place while it almost 

 entirely arrests effusion. This will have a very important bearing on the indication 

 of fire -damp, because it is now only necessary to fit a plate of Sicilian marble into the 

 iron funnel of fig. 915 in the place of the porous tile A, and that instrument may bo 

 used instead of the delicate instrument shown at Jig. 916, which, it must be admitted, 

 is extremely delicate and liable to destruction. By regulating the thickness of the 



marble from one-eighth of an inch to 

 one inch, Mr. Ansell meets the cases 

 of the varying times required for the 

 various rates at which gas accumulates. 

 Instead, however, of using the instru- 

 ment, fig. 915, which is proposed only 

 as a means of w a rnin g to those on the 

 spot, he wishes to place in the pits an 

 instrument shown as fig. 917, which is 

 about a third of the actual size. This 

 is made of a thick cup of cast iron, 

 screwed into a stand of the same metal, 

 and from the other side of which an 

 upright pillar rises to a height just 

 above the top edge of the cup, as will 

 be seen in the illustration. The top of 

 the iron cup is closed by a solid disc 

 of Sicilian marble three quarters of an 

 inch thick, shown at A, where the iron is 



torn away for tho sake of demonstration. The cup contains mercury as represented by j), 

 while between B and A is a space B, occupied by atmospheric air. If the cup bo examined 

 it will be seen that the mercury D flows by*a bent tube into tho upright pillar where 

 of course it assumes the same level as in the cup. In the event of fire-damp accu- 

 mulating around the cup (and this instrument is to be placed near the roof at a fitting 

 place in the pit) it at once diffuses through tho marble, and by tho law above ex- 

 plained accumulates in the space B, and thereby presses on the mercury D and causes 

 it to flow and rise into the pillar till it touches the needle c, and thus to complete the 

 circuit between the poles of a battery, and to ring a bell which may be placed milta 

 or yards off in tho viewer's office or in tho lamp-cabin. The distance between, tho 

 mercury and the needle c determines tho point of mixture between the gases of winch 

 warning should be given ; and this may bo fixed at above or below the point of explo- 

 sion at will, by regulating, by means of tho screw which passes through tho top, whore 

 a pin E is placed, to enable a man to regulate the distance in the dark by feeling tho 

 pin ; H is a valve which should bo securely closed as soon as tho instrument h;:- 

 steadily fixed in its proper position, while F and o are tho wires which terminate in a 

 cable going to the office, &c., so that twenty of theso instruments communicate with 

 tho one bell, just as in the case of tho telegraph bells at hotels, where the boll rings 

 and a number drops to show which instrument is communicating tho state of the pit. 

 All tho time tho valvo A of tho aneroid indicator is open, the instrument is 

 a barometer, therefore it indicates chango of altitude ; at tho spot one wishes to test 

 the valvo is closed ; a knowledge of this will preclude tho possibility of mistaking 

 change of altitude for gas. It is well to remember that this instrument is not 

 intended to detect gas : its inventor intends it to be used only to determine how much 

 gas exists in any place whore it has ln-<m sliov.n tu cxi.-4. A few intelligent obser- 

 vations will mako any man as perfectly acquainted with tho indicator aa with hig 

 watch, 





