176 



FOREIGN AND MISCELLANEOUS INTELLIGENCE. 

 I. MECHANICAL SCIENCE. 



1. PARABOLIC RIDGES FORMED ON MOVING WATER. 



M. PONCELET has made some curious observations on the form of 

 the ridges produced when a body is placed in a stream of water 

 flowing with uniform velocity. When a point is placed in the 

 upper surface of water flowing with uniform velocity, a great 

 number of ridges appear of a parabolical form. If a stream of water 

 spout from an orifice in a vessel, these curves have their vertices in 

 a line joining the point and the orifice. The summit of the first 

 parabola is at the point itself, and is the limit of all the others. The 

 number of ridges is indefinite; they are placed at distances which 

 increase with their distance from the point. These ridges become 

 less and less elevated according to their distance from the point, 

 til they vanish altogether. The ridges are perfectly stationary and 

 invariable in their figure, whilst the motion of the fluid remains the 

 same, and they cease to exist the instant the point is removed. If 

 the vein flow in a trough with vertical sides, the same phenomena 

 take place as if it were not so confined; and the ridges are 

 suddenly terminated by the sides without suffering in flexion or 

 reflexion. 



From these phenomena one might at first sight suppose that the 

 molecules of the current deviate from their natural course and follow 

 the branches of the curve. This, however, is not the case, as may 

 be proved by throwing fine powder on the liquid vein: the particles 

 of the powder cross these ridges, and follow the same course which 

 they would take if the ridges did riot exist, 



In plunging several points into the vein at different distances, the 

 same system of ridges is obtained for each point, and the curves 

 cross each other at the points where they meet, without their form 

 being in the least altered. When the velocity of the vein is below 

 twenty-five centimetres (ten inches) per second, the ridges be- 

 come imperceptible. They become more and more distinct as the 

 velocity increases. The number of ridges also increase with an in- 

 crease of velocity, the long branches approach more and more to 

 their common axis. The author remarks, that we have here an 

 accurate method of determining the velocity of a current by com- 

 paring the form of the exterior ridge with those given by experiment 

 with a current whose velocity is known. 



If the point be moved in a straight line along the surface of calm 

 water, we have exactly the same parabolic ridges as we should have 

 with water flowing with the same velocity as the moving point*. 



* Annales de Chimie, xlvi. p. 5. 



