224 46 



the mercury falls, and the manner in which Ihe mercury falls inlo the tube, are 

 distinguishing features of this pump. 



The outer diameter of the fall-tube is 8 mm., the inner diameter 4.5 mm. at 

 the top, but it rapidly narrows so that the inner diameter, 5 mm. from the top, is 

 only 4 mm. The fall-tube consists of many capillary tubings of different inner 

 diameters, as the following table shows. 



Inner diameter Length of the tube reckoned from tlie top 

 4 mm. 0.5 — 5 cm. 



3 » 

 2.5 > 

 2 » 

 1.3 » 

 2 » 



Of course, care was taken that the passage from one dimension to another should 

 be as even as possible. The fall-tube is consequently conical and the advantage 

 obtained with this form, will presumably appear with sufficient clearness from the 

 following description of the working of the pump. 



The uppermost end of the fall-tube is situated in the middle of a concentrical 

 glass bulb g, Fig. 11, with a diameter of 2.5 cm. The mercury is conducted through 

 the side tube s into the bulb g and fills the space around the fall-tube. On account 

 of the surface forces the mercury can stand higher in the space round the tube 

 than the brim of the tube, without flowing into it. By continued influx of mercury, 

 however, the surface forces cannot resist and the mercury suddenly falls from all 

 sides into the tube, and at the same time cuts olT the gas contained in it. As the 

 mercury Hows many times more rapidly away through the fall-tube, widened at 

 the lop, than it enters through the side tube s, the influx of the mercury in the 

 fall- tube is soon interrupted. Then the mercury in g is raised again and after a 

 short time a new volume of mercury falls, as just now described, into the tube. 



All the mercury that falls into the tube forms a coherent column, forcing the 

 gas by its weight down the tube. Owing to the conical form of the fall-tube, the 

 mercury column becomes longer the farther it comes down the tube, and therefore 

 it has still more power to drive the compressed gas downwards, and at the same 

 time there is less probability that the gas can escape back through the mercury. 

 At low pressure the resistance of the gas to the falling mercury is quite infinitesimal, 

 and the latter would therefore gain a great velocity, and consequently be liable to 

 disintegrate into small drops, unless the friction in the tapering tube diminished 

 the speed. The conical form of the tube also is conducive to keep the mercury 

 together in one body, for the friction is greater at the lower end of the column. 

 The mercury at the upper end of the column will therefore exert a pressure on 

 the mercury beneath. In my opinion, it is also necessary that the tube should 

 be narrow in the spot where the falling mercury coincides with that standing in 



