( 35 < 5 ) 
One Man may work an Engine like the Model, and 
bigger every way in the Proportion of a Foot to an 
Inch. 
As at every Stroke, 14 cyUndrick (or ii cubic) Feet 
of Air are driven in, or as many cubic' Feet of Damp 
fuck*d out, if the Axis of the Cranks be turn’d round 
60 Times in a- Minute, one Man, in that Time, may 
change the whole Air in a cubic Space, whofe Side is 
8 Feet-, and One Horfe, by working 24 Pumps with 
Half the Velocity, will eafily do 4 Times the Work of 
One Man. 
The Engines work with a great deal of Eafe, becaufe 
no Preffure of Atmofphere is to be remov’d ; only a 
Velocity to be given to one Sort of Air, to change it 
for another. ' 
Fire will not do in all Cafes, tho* in fome, it will 
draw foul Air out of Mines with Succefs ; becaufe feve- 
ral Sorts of Damps extinguifh Fire, and fome fulmi- 
nate, and are dangerous, when Fire comes near them ; 
and even in common ftagnant Air, Fire will not keep 
in long. 
I am fenlible, that large Bellows have fometimes been 
madeUfe of for this Purpofe ^ but they require a much 
greater Power to produce the fame Effed, and cannot 
have the Advantage of being immediately chang’d from 
Forcing to Sucking ^ neither are they fo cheap as» the 
propos’d Engine, which may be all made of Wood, ex- 
cept the Cranky which muft be of Iron, and the Barrels 
of very thin Copper. 
VII. A 
