of Processes in the Fine and Useful Arts. 163 



take the liberty of sending you a notice of the ultimate state 

 to which I have brought it. 



As soon as I became practically acquainted with the admi- 

 rable working of steam engines, I thought of forming a dou- 

 ble stroke air-pump ; and I believe, since mechanics have 

 turned their thoughts to air-pumps and such hke matters, they 

 all wonder why such an instrument is not used. In 1823, I 

 showed Mr Adie one which I had contrived, the mechanism 

 of which was such, that it worked as a double stroke pump 

 until the exhaustion was carried as far as possible in that way, 

 and then, by turning two stop-cocks, it was worked as a single 

 stroke pump, with a vacuum above the piston, into which the 

 rarified air from below was forced previously to expulsion to 

 the air. Mr Adie's opinion was, that the instrument was too 

 complicated for use ; and I soon came to be persuaded that 

 that gentleman was right, and moreover, that there was no oc- 

 casion for such affectation towards a vacuum, so long as it can- 

 not be denied, that though there were no barrier whatever op- 

 posed to the free expansion of the air in the receiver, it would 

 still remain full of air of uncompressed density, and, for any- 

 thing that could be affirmed to the contrary, might contain as 

 many atoms of oxygen and nitrogen as there are stars in the 

 Milky Way ; and that therefore experiments in Boyle's va-' 

 cuum, however perfect the removal of the pressure, were' 

 always conducted in the presence of oxygen and nitro- 

 gen, unless the oxygen had been withdrawn by chemical 

 means. The value of the air-pump, however, is so very 

 great, that if I have the good fortune to secure your appro- 

 bation for mine, I am sure you will forgive me for troub- 

 ling you with this letter. The piston rod, see Plate II. 

 Fig. 10, is hollow, and the two parts of the piston (which 

 must be for receiving the collars of leather, which, by 

 the way, when screwed in, ought to be turned upon a lathe,) 

 form a capsule for the lower extremity of the tube, having an 

 inversely conical opening cut in the region below. In this coni- 

 cal portion a common free conical valve (Scottice jumping 

 valve) plays in oil. One side of the piston and its collars of 

 leather is drilled to receive a hollow tube with its extremities 

 bevelled to a conical form, the collars of leather acting as a ' 



