Dr Buchanan's Description of an Instrument , fyc 133 



ed as a sober fact in Natural History, quite unconnected with 

 the gigantic exploits of the God Thor, or the fanciful absurdi- 

 ties of the Scandinavian mythology. We cannot suppose, that 

 the most ultra-sceptical can now continue to doubt with re- 

 gard to facts attested by such highly respectable witnesses. 



Art. XVIII. — Description of an Instrument for Extracting 

 and Condensing Air without the assistance of Valves or 

 Stop-cocks. By Andrew Buchanan, M. D. Glasgow. In 

 a Letter to the Editor. 



Dear Sir, 

 Observing, in a recent number of one of the Philosophical 

 Journals, an account of an air-pump -without artificial valves, 

 by Mr Ritchie of Tain, it brought to my recollection a model 

 of an instrument which I have had beside me for many years, 

 and of which, if you can spare a corner in your next number, 

 I shall be obliged by your inserting a description. 



The original form of the instrument, such as it has in the mo- 

 del just mentioned, is represented by Plate I. Fig. 11. It consists 

 of two cylinders of the same size, each furnished with a piston. 

 They are both open at the top, while at the bottom the verti- 

 cal one communicates by a small opening with the body of 

 the horizontal one, and the horizontal one itself communicates 

 with the receiver by means of a conduit pipe. 



The mode in which the instrument acts in extracting or 

 condensing air is easily perceived by the inspection of the 

 figure. Suppose both pistons at the bottom of their respective 

 cylinders, — let the horizontal piston be drawn up, the air of 

 the receiver will diffuse itself through the horizontal cylinder. 

 Next, let the vertical piston be drawn up, and the air will in 

 like manner diffuse itself through the vertical cylinder. The 

 horizontal piston is now pushed down, and after it the vertical 

 one. By this arrangement it is clear that the whole air con- 

 tained in the vertical cylinder has been forced out and perma- 

 nently excluded from the receiver ; for, on the descent of the 

 vertical piston, the air is forced from the vertical into the hori- 

 zontal cylinder, and being prevented from returning into the 



