j 4 The Uses of

pelled Air passes out at any Orifice: this Xtried by hanging a light Valve, which wasfix Inches long, and three and a half broad*over the Nose of the Bellows, by pliantLeathern Hinges, which was agitated andlifted up much, from a perpendicular to amore than horizontal Position, by theForce of the tussling Air. Whereas inanother ventilating Machine, composedof a Wheel with Fans, in a Drum, such asis described in Agricola de Re Metallicd ,and in the ^Philosophical Transactions,a like Valve was moved very little, by theForce of the Air which it conveyed ; whichevidently shews the great Difference thereis, in the Velocities, and consequently theQuantities of Air that are conveyed by theseMachines. There is another more accu-rate Way of estimating the Velocity ofAir, viz. by holding the Orifice C of aninverted Glass Siphon or Crane full ofWater ; such as is described, Fig. 9. CRI,opposite to the Stream of Air, whereby theWater will be depressed in the Branch C R,and raised in the other Branch I, in propor-tion to the Force with which the Water isimpelled by the Air. When this Crane

was