1S4 Dr Buchanan's Description of an Instrument 



conduit pipe, by the interposition of the horizontal piston, it 

 makes its escape by the only remaining passage at the top of 

 the horizontal cylinder. The different parts of the instru- 

 ment are now in the same position as at first ; the alternate 

 motion of the pistons may therefore be repeated and continu- 

 ed, till the exhaustion of the air in the receiver be as great as 

 required. 



As the process of condensing air is the reverse of rarefying 

 it, so it is performed by reversing the order of the motion of 

 the two pistons. Supposing, as in the former case, both pis- 

 tons at the bottom, the vertical one is first drawn up, and next 

 the horizontal one. They are then made to descend in the 

 same order, when it is obvious that a volume of air, equal to 

 the capacity of the vertical cylinder, will be forced through 

 the conduit pipe into the receiver. The pistons are now again 

 at the bottom of the cylinders, so that the operation may be 

 repeated as before. 



In the year 1817, in drawing up an account of this instru- 

 ment, I slightly modified its form. It is clear that there is no 

 necessity for the horizontal cylinder being so large as the ver- 

 tical one ; on the contrary, the great size of the former is a 

 disadvantage in working the instrument, without adding in 

 any respect to its efficiency. I therefore diminished the hori- 

 zontal cylinder both in length and diameter, and reduced the 

 instrument to the form represented by Fig. 12. 



I have not been able to devise any farther simplification of 

 the instrument itself. The process of working it, however, is 

 somewhat operose. This practical inconvenience may be ob- 

 viated in various ways. By the following apparatus, the 

 successive strokes of the two pistons are effected in the com- 

 mon air-pump by the partial revolutions of a wheel acting on 

 the piston rods. 



Fig. 13. is a delineation of this apparatus. It consists of 

 two simple instruments wrought by the same moving power. 

 The wheel has two quadrants opposite each other, furnished 

 with teeth acting on the corresponding teeth in the upper half 

 of the vertical piston rods. By this means, on turning the 

 wheel a quarter round, the two vertical pistons receive a si- 

 multaneous motion in opposite directions. The two horizontal 



