132 REPORT— 1879. 



piston with even greater accuracy than when the tuning-fork is otherwise 

 set in vibration. The reason is, that in other cases there is an irregularity 

 in the loudness of the sound which alters slightly the velocity of the 

 sound. 



In the small-sized instrument the tuning-fork is set in vibration by 

 means of a striker or rod, which is drawn by the hand between the prongs 

 of the tuning-fork (which approach each other at their extremities). A 

 little practice enables anyone to obtain in all cases the same loudness of 

 sound. The Committee have added to this instrument a circular scale 

 along which an index travels, being moved by a rack on the piston so 

 arranged that it cannot give a false indication. By this means the length 

 of tube can be read off easily, even in a bad light. In its present form 

 the instrument is easy of use and convenient, and cannot easily get out 

 of order. A thermometer is attached by means of which the small 

 temperature correction can be applied. The percentage of fire-damp is 

 read off directly upon the scale. 



The accuracy of the instrument is such that the percentage of fire- 

 damp can be determined with an error of considerably less than one per 

 cent. The Committee would draw attention to experiments described in 

 the ' Philosophical Magazine ' for April 1879, which show that a difference 

 of one part in 300 is not found between different observations of the 

 length of tube which resounds to a given tuning-fork. 



On August 25, 1879, the Committee were enabled to descend the 

 Wharncliffe Silkstone Colliery by the kindness of the manager, Mr. George 

 Walker, who accompanied them, with a few other gentlemen interested in 

 the experiments. This pit is at a depth of 200 yards. Mr. Walker had 

 kindly arranged to stop the ventilation of the pit at the end of the 

 workings, so after proceeding a mile through the galleries they came to 

 this spot, where they hoped to find a large amount of fire-damp. But 

 only a slight quantity was to be found ; the Davy lamp generally showing 

 but a feeble blue cap, and the Forbes' indicator registering only small 

 percentages. Disappointed here, they were taken by Mr. Walker to 

 another working, where it was thought possible that there might be some 

 gas. Here in a crevice in the roof a flow of gas was found forming a 

 stratum of light gas. Here the instrument indicated quantities gradually 

 increasing from 14 per cent, as the tube got filled with the air in the 

 crevice, up to 28 per cent. But the small quantity of gas rendered this 

 experiment unsatisfactory, and the Committee were then taken to a 

 disused part of the mine where it was known that there was a blower. 

 Here sufficient quantities were found, and the instrument registered gas 

 with more readiness than the Davy lamp. But the greatest quantity 

 registered was 6 per cent., or twelve times the smallest quantity which 

 the indicator detects. The fact is that there is in the present form of the 

 instrument a difficulty in filling the tube with the air of the place under 

 examination. The Committee consider that it would be well to alter the 

 instrument so as to obviate this difficulty ; and they also recommend that 

 experiments should be made to test whether the calculated percentages of 

 fire-damp agree with actual experiment. They have also to report that 

 the instrument was of a convenient form so as to be portable, and was 

 very consistent in its indications, and they can assert that this instru- 

 ment is capable of detecting and measuring fire-damp even in small 

 quantities. 



