112 PHYSIOLOGY OF THE CIRCULATION. 



The ascent of stagnant liquids observed in the 

 experiments of Venturi has very justly been referred 

 to atmospheric pressure, or, in other words, to dimi- 

 nished pressure witltin the tube traversed by the 

 stream ; and that this is the true explanation of the 

 phenomenon is proved by the following among other 

 reasons. 



1. To ensure the success of the experiment all the 

 joinings of the apparatus must be perfectly air-tight ; 

 otherwise the ascent of the stagnant liquid will be 

 very limited. 



2. The same stream which drew up a column of 

 water to the height of twenty inches, raised mercury 

 to the height of one inch and a half only, which 

 elevations bear to each other very nearly the same 

 proportion as the specific gravities of these two 

 liquids. 



3. The following experiment, performed by Hauks- 

 bee, at the commencement of the last century, is 

 interesting, not only for the clear light which it 

 throws upon the present question, but also from its 

 establishing for this country the credit of the first 

 discovery of this particular property of currents. 



A quantity of highly compressed air was made to 

 pass through a pipe, which by means of two lateral 

 branches communicated with the cisterns of as many 

 barometers, one of which was placed at the distance 

 of three feet from the main pipe. 



" The result of which was, that this rapid current 

 of air so lessened the pressure of the atmosphere 

 upon the stagnant mercury in the cisterns of the 

 respective barometers, that the mercury was nude 



