ON THE ARTERIES. 351 



from what it will do if forced in any other direc- 

 tion, by a vis a tergo. When water is poured out 

 of a vessel, it acquires velocity as it descends ; the 

 column is largest above and drawn fine below, 

 because it is increasing in velocity, and the stream 

 that has a greater velocity must be smaller in 

 diameter : but continuing to descend, the stream 

 acquires such a degree of velocity, that the atmos- 

 phere offers resistance, and then it again spreads 

 out. But a column of water sent upwards as a 

 jet d^eau, instead of contracting as it ascends, en- 

 larges. The fluid is retarded as it mounts ; and 

 the stream being still propelled from below, it is 

 forced between the filaments of the column above 

 and dispcistus ilieiii, eiilcirgiiig the column as it as- 

 cends, and giving it a conical form. Hence it fol- 

 lows that if water is to be discharged from a re- 

 serv^oir along a horizontal tube, it will flow more 

 rapidly if the tube be a cone, with its lesser end 

 inserted into the reservoir ; for the weight of the 

 water in the reservoir still being a vis a tergo, and 

 the stream from behind forcing itself between the 

 filaments of the column and so dispersing them, it 

 is clear that the increasing diameter of the tube 

 will correspond with the natural enlargement of 

 the column of water, and give it an unimpeded 

 exit. Here, then, we have another explanation 

 of the increasing diameter of the arteries to re- 

 ceive the blood propelled by the heart. 



In laying pipes for ?ijet d^eau, the ascent of the 

 water will be diminished by any sudden angle in 



