1 16 ConftruBien of an Air Pump without the Lower Valve, 



The head of this pump is not divided as the common one is, to diflodgc the teeth fef the 

 wheel fvom the piflon rods when the pump is to be taken apart, but is made whole, except 

 a fmall piece in the back, where the wheel is let in ; which makes it much more convenient 

 to remove the head, or place it on the barrels. The wheel is freed from the pift.o;i rods 

 when required, by pufliiiig it into the back part of the head ; and when it is drawn into its 

 place and conneifVed with them again, a button is fcrewed into the focket of the axis be- 

 hind, to keep it in its place. This makes the head tefs troublefome to remove; but its 

 chief ufc is to diflodge the pifton rods from the wheel, that they may be put down into 

 the cifterns when tlie pump is not in ufe, where they will fland uncomprefled, and retaia 

 their elafticity better than if kept in the barrels. In thcfe cilferns they may alfo (laud co- 

 vered with oil, if ncccffary, as they are large enough to admit of it. 



The principal joints of the pump are funk in fockets, that the leathers which clofe them 

 may be covered with oil to prevent leaking*. 



For convenience, the lower part of the pump is fitted with drawers to contain the appa- 

 ratus. A door opens behind one range to a place referved the whole height, to get at the 

 under part of the receiver plate, and fix apparatus to it for fome experiments. In this 

 place ftand the long tubes, and fuch tall glalfes belonging to the apparatus as will not go 

 into the drawers. The barrels. Sec. of the pump are covered with a cafe or head, which 

 keeps them from dud and accidents when the pump is not in ufe. The apparatus is fecured 

 between Aiders, &c in the drawers, fo that the whole machine may be cafily removed in one 

 body, without danger. 



As there was no glafs manufa£tory in Mr. Prince's vicinity, he fent to Europe for his 

 apparatus ; but unluckily the gauges, with fome other parts, had not been forwarded to him 

 when he wrote his account. He had only a fmall tube of two-tenths inch bore, which he 

 ufed as a common gauge ; but this was not fuflicient to determine the power of the pump. 



All that he could fay of the iiiftrument was, that he found it much more convenient to 

 ufe than one of the common fort ; that it would exhauft a receiver much fooner, and keep in 

 order much longer, for being made without valves, which mud depend on the fprlng of 

 the air to open them. When a common pump in his pon'efTion had been fitted up with 

 valves, leathers, &c. at the fame time with this, the valves of the common pump have be- 

 come too dry and Riff to ufe, while this pump has continued in good order. He attributes 

 this, in part, to the moifture which the valves on the top plates receive from the piftons 

 every time the pump is ufed; the piftons being always kept moiflened with oil in the cif- 

 terns, where they Hand when the pump is not in ufe ; and in part to the power which the 

 piftons have over thefe valves, by condenfing the air againft them. 



Fig. 6, Plate \ I, rcprefents a perpendicular feiSlion of one of the barrels, the two cifterns, 

 condenfing gauge, &c. where AB rcprefents the barrel ; CD is theciftein on which it ftands; 

 a a a a the leatliered joint, funk into a focket and buried in oil : EF is the pifton ; the cylin- 

 drical rod paiTmg through a collar of leathers, GG, in the box HI. K ft\ews the place 

 of the valve on the top plate K, covjSed by the crofs piece MM, into which the pipe OO 



\ This, I find, is very cfTcAual 1 having never known one of the joints reciircd in this way to lc3k, though 

 the pump has ftood for a long time ; whereas a ponable pump which I have, made by Mr. Nairoc, London, has 

 leaked, and repeatedly been refitted with new oiled leathers in the fame time. F. 



