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Of the Air-Pump .

• • There is a glass tube Immmn open at both ends, and about 34 Inchesioug the upper end communicating with the hole in the pump-plate,and the lower end immersed in quicksilver at n in the vessel N. Totliis tube is fitted a wooden ruler mm> called the gage, which isdivided into inches and parts of an inch, from the bottom at n(where it is even with the surface of the quicksilver) and continuedup to the top, a little below /, to 30 or 31 inches.

As the air is pumped out of the receiver M } it is likewise pumpedout of the glass tube Imn, because that tube opens into the receiverthrough the pump-plate ; and as the tube is gradually emptied of air,the quicksilver in the vessel N is forced up into the tube by thepressure of the atmofphere. And if the receiver could be perfectlyexhausted of air, the quicksilver would stand as high in the tube, *as it does at that time in the barometer : for it is supported by thefame power or weight of the atmofphere in both.

The quantity of air exhausted out of the receiver on each turn ofthe handle, is always proportionalste to the afcent of the quicksilveron that turn ; and the quantity of air remaining in the receiver, isproportionalste to the defect of the height of the quicksilver in thegage, from what it is at that time in the barometer.

I fhall now give an account of the experiments made with the air-pump in my lectures; fhewing the resislance, weight, and elasticityof the air.

I. To ß:ew the reßßance cf the air.

Fig. 3. 1. There is a little machine, consisting of two mills, a and

which are of equal weights, independent of each other, and turnequally free on their axes in the frame. Each mill has four thin armsor fails„ fixed into the axis: thofe of the mill n have their planes atright angles to its axis, and thofe of b have their planes parallel toit. Therefore, as the mill a turns round in common air, it is butlittle resisted thereby, because it’s fails cut the air with their thinedges: 'but the mill b is much resisted, because the broad sides of it’sfails move against the air when it turns round. In each axle is a pinnear the middle of the frame, which goes quite through the axle,and stands out a little on each side of it: upon thefe pins, the flider dmay be made to bear, and io hinder the mills from going, when theArong spring c is fet on bend against the opposite ends of the pins.

_ Having fet this machine upon the pump-plate LL (Fig. 1.) drawup the flider d to the pins on one side, and fet the spring c at bend. upon