MINING. 301 



very interesting series for study, but will probably be given with 

 full minuteness of detail under the head of military apparatus. 



VENTILATION OF MINES. 



Although the machines and apparatus for promoting the actual 

 ventilation, scarcely come within the limits of the present exhibi- 

 tion, a number of instruments employed for testing its condition, 

 or measuring the actual volume of air in circulation, will consti- 

 tute an interesting group for study and comparison. 



Years ago, and a whiff -of powder, or tobacco-smoke, was 

 watched as it floated along a measured length of level in the 

 mine, and then, multiplying the velocity of the current, as thus 

 determined, by the average area of the passage, the total amount 

 passed in a given time was calculated. Mr. Biram, of Earl 

 Fitzwilliam's collieries, appears to have been first to devise a 

 suitable mine anemometer. A number of vanes of a light mate- 

 rial, segments of a screw, are arranged upon a horizontal axis in 

 such wise, that being presented broadside to the current of air, 

 the wheel shall make a certain number of revolutions, for so many 

 feet of air passing through it. Thus, if the pitch of the screw be 

 2 feet, whilst the air passes through 2 feet, the screw or wheel will 

 make one revolution. The number of revolutions, or correspond- 

 ing number of feet, is registered by indices as a series of circles, 

 and thus on the larger instruments, made 12 inches in diameter, 

 and, indeed, recently in the smaller ones of 6 inches, 4 inches, 

 and even 2 inches in diameter, a continuous self-acting record may 

 be obtained, up to 10,000,000 feet. A vast number of these 

 instruments have been supplied by Messrs. Davis and Son, of 

 Derby ; those intended for brief experiment are made to register 

 only a small number; but those which have several dials are 

 intended to record the revolutions for hours and days together, 

 and they are previously tested by the makers at different velocities, 

 and provided with a table of corrections for error. 



A small and delicate instrument on a very similar plan is that 



