Mr Dunn's Description of an Improved Air-Puvip. SSd 



mentioned my views on the aul^ect to severtl gentlemen quaii- 

 Ged to judge of their correctness^ and soon had an opportu« 

 iiity of putting them to the test of experiment I received an 

 order to make one for Mr Lees, lecturer on mechanical )>liik>so- 

 phy in the School of Arts here, on condition that he was to be 

 permitted to return it, if, on trial, it was not found capable of 

 executing all tluit I had taught him to expect. This pump, 

 through the kindness of Mr Lees, in whose poesesskm it has 

 been for the last eighteen months, was exhibited to the Society 

 iivc the Improvement of the Useful Arts, on 19th December 

 18^*. 



That the peculiarities of the construction of my pump may 

 be more readily perceived, I shall first shortly describe the com- 

 mon construction, and then its most perfect form, as improved 

 by Cuthl>ertson. 



The common air-pump consists of two barrels A A', Fig. 1., 

 Plate IV, in which the pistons PP' are fitted and moved by 

 the racks RR^ and pinion O, the pistons being thus raised and 

 depressed alternately by turning the \vinch W. In the bottans 

 of tlie barrels there are openings, communicating with the re- 

 ceiver or bell-glass ; over these openings valves of waxed silk or 

 bladder are so placed as to admit of the passage of the air from 

 the receiver through them, but to oppose its passage from the 

 barrels to the receiver. It is obvious, that, on dravnng up 

 cither of the pistons, a vacuum will be formed under it till the 

 air from the receiver, by its elastic force, opening the valve V 

 or V, distributes itself equally betwixt the barrel and receiver. 

 Now, as the pistons are funushed with valves PP' of the same 

 kind) and opening in the same direction as VV, on pushing 

 either piston down, the air in the under part of the barrel lietng 

 prevented by the valve at the bottxKU from returning to the re- 

 ceiver, will open tlie piston- valve and esca]x; into the apartment, 

 with the air of which the piston-i-alve oomnmnicates ; and these 

 effects will fellow the raising and depressing of the pistons, at 

 long as the air in the receiver has sufficient elasticity to open 



• 'Oie hi5trnment had been previously submitted to the examination of 

 Dr Turner, one of the Secretaries of the Society «f Arts, wtw reported that 

 be bsiA miuutely eKaoumxl it, and was perfectly astisfied with its perfonnaaoe. 

 On his representation to the Council of the London University, I have since 

 received an order to make xme fbr the chemical class of that institution. 



