FIRE-DAMP INDICATOR 



397 



shilling. If, when the instrument be ready, gas impinge on the porous tile diffusion 



takes place, and the pressure of the .accumulating gas forces the mercury against the 



platinum-pointed wire, and the circuit 



being thus completed, telegraphic warning 



is given on the spot, as well as in the 



manager's room, if such be desirable, 



either by a needle or by a bell. 



In the case of known accumulations, 

 Mr. Ansell considered that, if the amount 

 per cent, could be readily ascertained, 

 measures in proportion could be taken 

 to sweep out by ventilation such a dan- 

 gerous mass. He tried instruments upon 

 many systems, all acting by diffusion, and 

 with many obtained splendid results, such, 

 for instance, as with a column of water 

 or of mercury ; but in all cases tempera- 

 ture would be fatal, unless in the hand 

 of a scientific man. Then, again, there 

 is another action depending on the ex- 

 change of the proportions of the gases 

 composing the atmosphere, which leads 

 to errors ; he has, therefore, been led to 

 use the aneroid barometer, which, al- 

 though far from a perfect instrument, is 

 still reliable, and may be depended on till 

 a better is discovered. In this place we 

 may mention that Mr. Ansell is now at 

 work at a most beautiful pocket indicator, which we have reason to believe will avoid 

 all the difficulties of the aneroid, and be simply an indicator, not a compound instrument 

 answering three purposes, as does the aneroid indicator. That marvellous arrange- 

 ment, the Davy lamp, gives magnificent indications, and it may be asked, why try 

 to go beyond it ? Mr. Ansell says, ' I hope to supplement it by another indicator, 

 for it must still be used with my proposed instrument ; yet I am not without hope 

 that I shall soon make my instrument self-registering ; then I shall hope it will be 

 even more useful than in its present form.' 



The instrument, fig. 916, about to be explained, is an 

 aneroid barometer of the most delicate construction, the 

 brass back of which has been removed, and its place 

 occupied by a porous tile, B. There are besides one or 

 two minor alterations, such, for instance, as a small 

 valve. A, to be used at pleasure. This instrument acts by 

 pressure on the outside of an aneroid chamber, c, which, 

 by a system of levers, causes a hand to travel over a dial 

 face, which is graduated in inches just as the ordinary 

 aneroid, so that it can be used as a barometer, and as 

 such can be relied upon. If we desire to experiment for 

 fire-damp, or for choke-damp, by means of this indicator, 

 i* must be taken into the neighbourhood of the suspected 

 atmosphere, and held by the ring handle, A, till it has 

 become of the same temperature as the new place. It is 

 absolutely necessary to follow these directions, because, 

 when the valve is closed, the instrument is affected by 

 temperature; when the temperature is equalised, which 

 is usually in a short time, it may be known by the fact 

 that the hand remains stationary after the valve is closed. 

 The valve, which is simply a hole bored through the 

 handle A, so that by turning the handle the atmosphere 

 is admitted or excluded at will, is closed by screwing it 

 tightly, and the position of the hand recorded. Then 

 the brass cap D, which protects the porous tile, is removed, 

 and the instrument held up into the suspected atmo- 

 sphere, when in about 45 seconds the maximum effect is 



produced ; at this time it is necessary to read the barometer accurately, because, the' 

 maximum point having been reached, effusion takes place, and the hand travels back 

 to zero. 



Effusion is the mechanical passage of gas through the tile by pressure alone. This 



