﻿and Fluctuations. 345 



nearest approach to a perfect vacuum may be obtained, as is in 

 fact done by SprengePs air-pump. 



But though this principle of fluids has long been known and 

 admitted, it seems to have been regarded rather as a scientific 

 curiosity than as affecting the everyday relations of life. One 

 familiar instanee will show that such a view is very erroneous. 

 Certain gratings in the streets cover shafts which lead directly 

 to the main drains, and it might be supposed that through these 

 foul smells would be very apt to escape into the open air; but 

 the flow of the water and the air which it drags with it through 

 the drain prevents this ; it continually rasps off the bottom of 

 the atmospheric column and causes a descending current of 

 air through the shaft, so that no vapours from below can come 

 up. The force of this descending current can easily be seen 

 by holding a lighted torch near the grating of such a shaft 

 and watching how the flame is sucked in — quite as strongly 

 sometimes as it would be near the bottom of a doorway opening 

 into a heated room; and indeed the power of friction, acting 

 in a similar way, was tried a few years ago in the ventilation of 

 coal-mines. A number of jets of steam were driven forcibly into 

 the upcast shaft, and these, dragging the air with them, gave 

 rise to a corresponding descending current in the downcast shaft. 

 In practice, however, the method was found expensive, and has 

 been gradually given up even by those who were at first very 

 much in favour of it*. 



From a meteorological point of view, the importance of this 

 principle was, I believe, first brought forward by Sir Henry 

 James f, who established the fact that in the heavy squalls of a 

 gale of wind a barometer on the lee side of a wall had a lower 

 reading, and one on the weather side a higher reading, than a 

 third barometer placed without obstruction in open ground. 

 More recently Professor Wild, of Bern, has shown that a south- 

 erly gale blowing over the Alps and passing as an upper current 

 over the valleys of the north-eastern part of Switzerland, more 

 especially of the cantons Uri, Glarus, and Schwytz, draws the 

 air of the confined valleys away with it, so as to cause a very 

 marked depression of the barometer — and that in consequence 

 air from above, which in passing over the mountains has been 

 drained of its moisture and warmed by the heat of condensation, 

 descends as a dry hot wind, which is almost peculiar to these 

 localities, and is there known as the FohnJ. 



* For a full discussion of the merits of this system see "Experiments on 

 the relative value of the Furnace and the Steam-jet in the Ventilation of 

 Coal-mines," communicated to the North of England Institute of Mining 

 Engineers by Nicholas Wood, President of the Society. Newcastle, 1853. 



t Trans, of the Roy. Soc. of Edin. vol. xx. p. 377- 



j Dr. H. Wild, Ueber Fbhn und Eiszeit, 1868, p. 30. 



