192 The Ventilation of Coal Mines, [April, 



to a considerable extent in France and Belgium, but does not 

 appear to have been successful. More recently, however, this form 

 has been revived, with the important modification of placing the 

 cylinders, or rather air-chests — a box of square section being usually 

 employed— horizontally instead of vertically, so that the valves may 

 be made of large sectional area without any counterbalance ; the 

 pressure against the seat when closed being very much less than 

 is the case in the vertical cylinder. Prominent among this class is 

 that known as Nixon's machine, which has two single-acting boxes 

 and pistons, each 20 feet square, and making a stroke of 30 feet. 

 It is of comparatively recent introduction, and is mostly used in 

 South Wales. The essential conditions for the successful construc- 

 tion of a machine of this class are great lightness in the working 

 parts, and the use of guide rollers to keep the weight of the piston 

 from wearing out the packing unequally. 



Fabry's pneumatic wheel and Lemielle's ventilator are examples 

 of machines with cylinders and rotating pistons. The former 

 consists of three horizontal fans, each with three blades, suspended 

 in a pit bounded by walls, forming segments of circles. The air is 

 scooped up by the advancing blades in passing these portions of the 

 pit, and is prevented from returning through the unenclosed 

 portion, by a system of interlocking plates at right angles to the 

 main air- way on the opposite sides of the shaft, xllthough a com- 

 paratively simple contrivance, it is difficult to express its mode of 

 action clearly without illustrative drawings. The amount of air 

 discharged per revolution is comparatively small, with reference to 

 the size of the cylinder described by the points of the blades ; but, 

 as it is driven continuously? a steady current can be kept up by 

 working it at a moderate rate of speed. 



Lemielle's machine may be best described as a common feather- 

 ing paddle-wheel, with only two or three float-boards, which is 

 placed eccentrically, and made to revolve in a circumscribing 

 cylinder, provided with intake and discharging passages nearly 

 opposite to each other, the axis of the wheel being placed vertically 

 instead of horizontally. The blades corresponding to the float- 

 boards in the paddle-wheel are kept, by their eccentric rods, in close 

 contact with the wall of the cylinder as the axis revolves, from the 

 moment they pass the intake until they reach the discharging port, 

 where the volume of air included between two following blades 

 passes out into the atmosphere. 



The usual dimensions of these machines are as follow : — 

 Diameter of the drum, about 10 feet; of the outer cylinder, 13 feet ; 

 length of blades, 7 feet ; when making from 20 to 30 rovolutions 

 per minute, the effective difference of pressure produced is from 

 J to 1^ inches of water. 



In the second, or centrifugal, class of ventilators, fans of all 

 kinds have been adopted at different times, both in this country 



