112 REPORT—1849. 
affect the furnace, and subject to visible inspection on the surface by the constant 
application of the water-gauge. 
The machine erected by Mr. Powell, at Gelly Gaer Colliery, was for the express 
purpose of ascertaining its power of rarefaction and general applicability to the 
purpose of mine ventilation, in order to its application to his larger and more ex- 
tensive collieries. The trial has been made, and a letter expressive of his entire 
satisfaction was produced. 
The machine is applied to the colliery by an air culvert connected with the upcast 
shaft, which of course is closed at the top. The diameter of the drum is 22 feet, 
the radial length of the compartments 6 feet, leaving the central space 10 feet dia- 
meter: 16 feet being the mean diameter of the radial compartments, the centrifugal 
force at 120 revolutions per minute is 39°25, which, multiplied by the weight of 
6 cubic feet of air = 444; of a pound, will give or produce a rarefaction to 17°5 lbs. 
on the square foot in the upcast shaft. The following table expresses the rarefac- 
tion consequent upon the velocities stated :— 
Water Mercury 
Ibs. in inches. in inches. 
60 revolutions per minute 4°3 per square foot 0°81 
90» ” hth 8 1°8 “13 
120 » 2 17°3 rf 3°3 24 
150, » 27-0 pa 5°2 "38 
180 a, a? 39°0 ” 74 “55 
210 ”» ” 53°0 a 10°0 ‘73 
These, though calculated as above, are most satisfactorily corroborated by actual 
experiments. 
It will be necessary to draw attention to two conditions in which this machine 
may be placed; first, to that when unconnected with any pit or air course, viz. air 
from the atmosphere having free access to the centre and space for free discharge 
from the periphery, and a velocity given to it of 130 revolutions per minute, creating 
a rarefaction of 17 lbs. per square foot in the middle cf the drum, then the velocity 
of the air through the machine would be 108 feet per second, and the aggregate 
quantity discharged 8424 cubic feet per second, or 505°440 feet per minute: in 
this (first) condition, the whole power is absorbed in displacement of air. The 
second condition is the very reverse of the first, viz. when no air is permitted to 
enter the central part, and of course none can be discharged at the circumference ; 
the steam-engine exerting the same force as before, thus relieved from the resistance 
of the air passing through the machine, expends its power in increasing the velocity 
of the drum, thereby creating in this case an extreme or maximum effect in rare- 
faction. A due consideration of these opposite conditions proves that the resistance 
of the machine, or the power required to turn it, will be in proportion to the quan- 
tity of air ascending the upcast shaft, and the amount of rarefaction required to 
draw it through the colliery ; and the principle of self-adjustment being such, that 
if from any cause a less quantity of air is passed through the colliery, the steam- 
engine (exerting the same power) will of itself accelerate the velocity of the drum, 
and thereby increase the rarefaction. For the power applied being the same, the 
effect will be commensurate, in the quantity of air discharged, the amount of rare- 
faction attained, or in a compound of both. 
In contrasting this machine with the furnace, the author first notices, that it is ap- 
plicable to any depth of shaft. 
That it is not sensibly affected by change of atmospheric temperature or the fall 
of the barometer, but, on the other hand, by an increase of velocity at such seasons, 
may be made to obviate or counteract the danger connected therewith. 
- The uniformity of rarefaction maintained will also produce an uniformity of ven- 
tilation through the respective districts of the colliery, such as cannot be effected 
by furnace ventilation. 
. There will be no necessity for a stone drift to avoid the ignition of gases by the 
urnace, 
ae injurious effect of heated air upon iron in the upcast shaft will be prevented 
entirely. 

