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pinion is a crank to move the ventilator, and alfo a 
fly fixed to the end, to help the motion of the crank, 
which in the model is turned three times for each 
ftroke of the lever, and may be increafed or dimi- 
nifhed, according to the number of teeth in the 
pinion. 
The number of teeth in the great wheel moved 
by the lever is fixty-fix ; but need not have teeth 
above half way round. 
The wheel fixed to the rochet has thirty- three 
teeth, and its pinion eleven. 
The wheel fixed on the arbor, on the outfide, has 
twenty-four teeth, and its pinion fixteen. 
The wheel, which turns the fly, has ninety teeth, 
and the pinion turn’d by this wheel ten. 
The greater the number of teeth in the rochets, 
the better. 
This machine may alfo be applied to other ufeful 
purpofes at mines ; and it may be eafily made to 
turn a mill to grind corn ; or to turn a wheel to raife 
coals, or whatever elfe is wanted to be raifed fiom 
the mines. As I have not met with any thing of 
the kind defcribed, I take the liberty of defiring you 
to lay it before the Society j and I hope it may be 
made fome way ufeful to the public. 
I am, Sir, 
Your moft obedient humble Servant, 
Poland-Street, 
June 7th, 1758. 
Kea. Fitz-Gerald. 
Vol. 50. 
Explana- 
