ON AN INSTRUMENT FOR DETECTING FIRE-DAMP IN MINES. 131 



polished specimen of the iron exhibits the Wiedmanstiittian figures very 

 finely. 



A paper by Professor Giimbel, of Munich, entitled ' Die in Bayern 

 gefundenen Steinmeteoriten ' (' Sitzber. der K. Bayer. Akad. d. Wissen- 

 schaften, math.-phys. CI., 1878, 1) treats of the meteorites of Mauerkirchen, 

 Eichstadt, Massing, Schonenberg, and Krahenberg. He gives their 

 history, their earlier analyses, and includes some new analyses, and a 

 plate showing the microscopic sections as seen by the microscope. 



Report of the Committee, consisting of Mr. David Gill, Professor 

 G-. Forbes, Mr. Howard Grubb, and Mr. C. H. Gimingham, 

 (with potuer to add to their number), appointed to consider the 

 question of Improvements in Astronomical Clocks. 



This was only a preliminary Report, and at Mr. Gimingham's request 

 its publication is delayed until next year. — [Ed.] 



Report of the Committee, consisting of Professor G. Forbes (Secre- 

 tary), Professor W. G. Adams, and Mr. W. E. Atrton, appointed 

 for the purpose of improving an Instrument for detecting the 

 presence of Fire-damp in Mines. 



This instrument is intended to measure the quantity of fire-damp in a 

 coal mine. From the rough model shown by Professor George Forbes 

 last year, the Committee have constructed two new instruments, which 

 appear to them to answer their purpose quite well. The one is of a large 

 size, and is worked by an electric battery, and is rather expensive. The 

 other is small, portable, easily worked, and answers all the purposes for 

 which it is required. Both instruments are founded upon the facts, that 

 sound travels quicker in light gases than in dense ones, and that air which 

 is contaminated with fire-damp is lighter than pure air. The velocity of 

 sound in different qualities of air is compared by noting the lengths which 

 must be given to a brass tube to cause it to resound to a tuning-fork. 

 The length of tube is proportional to the velocity of sound. The instru- 

 ment consists essentially of a tube with a tuning-fork at one end of it, 

 and closed at the other end by a piston which can be moved in and out so 

 as to lengthen or shorten the tube. The tuning-fork is caused to sound, 

 and on moving the piston in and out the sound is heard to augment and 

 diminish according to the position of the piston in the tube. The piston 

 must be left in tbat position which gives the loudest sound. The length 

 of the tube under these conditions measures the velocity of sound, and 

 thence the percentage of fire-damp in the air. 



In the large-sized instrument the tuning-fork is kept in vibration by 

 an electric current which is made and broken in each vibration acting on 

 an electromagnet so as to maintain the vibrations. The Committee 

 have been unable to arrange the contacts in such a manner as to prevent 

 the occurrence of a false note of considerable loudness. But in spite of 

 this the ear can detect the true note and regulate the position of the 



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