﻿444 Prof. Swan on Bianchi's Air-pump. 



freely within the hollow piece E, and thus the cylinder C has 

 perfect liberty to oscillate without necessarily twisting the con- 

 necting tube H, although, owing to the unavoidable friction 

 between the rubbing surfaces, at each stroke of the pump a slight 

 torsion of the connector actually does take place. In order to 

 ensure the connexions remaining perfectly air-tight, oil-cups, 0, 

 are placed on Fthe pieces E, F, and a groove, G (fig. 2), is cut 

 round each of the cones D, so that when a little oil is dropped 

 into the cups it runs down into the grooves and effectually pre- 

 vents the passage of air. In the apparatus as constructed, 

 wings, W, iv, were added at the ends of the connector, and also 

 placed on the tubes A and B with the intention of stretching- 

 vulcanite bands over the wings to maintain the connexions until 

 such time as the atmospheric pressure, owing to the progress of 

 the exhaustion, should of itself be sufficient for that end. The 

 wings have proved convenient in handling the connector ; but it 

 has not been found necessary in practice to make use of vulca- 

 nite bands. A pair of stops might be attached to the pump- 

 stand immediately below and in contact with the wings, W, so as 

 effectually to hinder their oscillation. Thus, were it deemed de- 

 sirable, the slight torsion of the tube H, already noticed, might 

 be prevented. It is indeed quite possible that, failing this pre- 

 caution, in course of time the tube may become brittle by con- 

 tinued twisting. Not only the wings, but, I believe, the oil-cups 

 also, or at least that at B, might be dispensed with ; but I am 

 not disposed to advise that either of the cups be omitted. 



The apparatus, as described, which has been exceedingly well 

 constructed by Messrs. Kemp of Edinburgh, was designed by 

 me for use with a moveable air-pump which is placed on the 

 floor, while the pump-plate stands on a table. If the air-pump 

 be permanently screwed to a lecture-table, which, where room 

 can be spared, is a good plan, the apparatus might advantageously 

 be simplified. The helix might probably be made of a shorter 

 tube, and the fastening at B might be permanent. A straight 

 tube might even perhaps replace the helix, or a tube bent at 

 right angles, where the table does not admit of the pump- 

 plate standing in front of the pump. In either case suffi- 

 cient play might be given to keep the cone D in its place 

 while the pump is working, if the pump-plate were mounted 

 on a board with small castors or rollers; and, if necessary, 

 vulcanized bands might be stretched over the wings on the 

 tubes A, E, to draw them together. With a permanently fixed 

 pump, I would also suggest that, by making the helical connector 

 long enough, the connexions at A and B might be made abso- 

 lutely rigid and permanent, the oscillatory motion of the cylinder 

 being rendered possible simply by the torsion of the helix. Per- 



