304. =. Theory of the Pneumatic Parador. 
the orifice with those which are moving towards it,” becomes con- 
tracted, and then again enlarged; so that at the distance from the 
orifice equal to its semidiameter, where the greatest contraction 
takes place, the diameter of the stream is about eight tenths, and 
of course its area about two thirds of that of the orifice. Sir 
Isaac Newton gave the contracted part of the jet the name of 
vena contracta. The same phenomenon occurs when a cylin- 
drical tube is adapted to the orifice. It will be perceived, there- 
fore, that in the experiments of Venturi just described, the water 
did not entirely fill the tube at the part where the glass tube was 
inserted. Now flowing liquids in contact with air ina state of 
rest, carry along with them a portion of the contiguous air, and 
this effect taking place within the tube at the place of the vena 
contracta, the air in the glass tube becomes rarefied, and its elas- 
ticity being thereby diminished, is no longer sufficient to resist 
the atmospheric pressure upon the colored water in the vessel 
T, which consequently rises. Venturi, well aware, without 
doubt; though I do not recollect that he states the fact, that the 
ascent of the colored water was dependent upon the vena con- 
tracta, describes no attempt to obtain a similar result by inserting 
the glass tube at any other part of the tube KLV, and he was fat 
from applying to the whole tube, like some recent writers, a con- 
— true of only a small portion of it. 
- The only direct evidence, as it respects liquids, of the proposi- 
tion of Mr. Spencer, that has come to my knowledge, is a single 
experiment of Bossut, described in his work entitled “ Traité 
Théorique et Epérimenthl ad’ Hydrody- Fig. 3. 
nhamique,” the first edition of which was 
published in the year 1771. The follow- * 
ing translation from the edition of 1796, 
comprises Bossut’s description of the ex- 
periment, and his remarks thereupon. 
Let the cylindrical horizontal tube EN 
be adapted to the reservoir ABCD; and 
let us suppose this reservoir to pe’ kept 
constantly full to the height AB, and the 
| to <eeh tt in the tube without 
ee the pressure which re- é 
the weight of the column of water EN, the tube expe- 
