Ritchie on a Double-acting Air-Pump. 289 



the lower valve. To the lower end of the rod is fixed the 

 conical metallic valve V, which is allowed to rise and fall about 

 the tenth of an inch. A bent tube, T, connects the upper and 

 lower divisions of the barrel formed by the solid piston. This 

 tube is continued from the top to the plate of the air-pump. At 

 the entrance of this tube into the upper end of the barrel, is 

 placed a valve of oiled silk, opening inwards, to allow the air 

 from the receiver to expand into the upper part of the barrel 

 when the piston is depressed. Two valves, either conical or 

 of oiled silk, are placed on the upper and lower ends of the 

 barrel at F, E opening outwards, to allow the air in the barrel 

 to escape into the atmosphere. When the piston is depressed, 

 the conical valve shuts the communication between the barrel 

 and the receiver, and the air is forced out at the valve F, whilst 

 the air in the receiver rushes into the space above the piston 

 to supply the vacuum thus formed. When the piston begins 

 to rise, the conical valve, on the end of the brass rod, is raised, 

 and the air from the receiver follows the piston till it has 

 reached the top of the barrel, and expelled the air through the 

 valve E. The next depression of the piston performs a similar 

 office, and thus the full of the barrel of air, of the same density 

 as that in the receiver, is at each stroke expelled, and conse- 

 quently the exhaustion will go on with twice the rapidity of 

 that produced by a single barrelled air-pump of the same size. 

 Instead of the hollow tube for the piston-rod the wire 

 might be made to pass through the piston, as in the French 

 construction, with two conical valves on the extremities ; but 

 the construction I have described seems to me the least liable 

 to objection. 



ON THE METHOD OF OBSERVING THE FIXED LINES 



IN THE SOLAR SPECTRUM. 



BY J. T. COOPER, Esq. 



AS many with whom I am acquainted have sought in vain 

 ~^ for those dark bands which occur in the solar spectrum 

 produced by prismatic refraction, usually known by the appel- 



