OF ARTS AND SCIENCES. 



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borders of the circular plane, where its edge is thin, but it follows 

 more or less the direction of the curve of the edge, if it is thick and 

 rounded.* (Savart, Ann. de Chim. et de Physique, Tom. LIV. p. 119.) 



" 3, When a jet of air impinges upon a surface of limited extent, the 

 atmospheric pressure upon the opposite side of the surface, in conse- 

 quence of the lateral communication of motion, is diminished, and a 

 current will be established through a tube, one of the extremities of 

 which is placed in the point of diminished pressure, and the other be- 

 yond the borders of the surface. This is the important principle upon 

 which the efficiency of ventilators and chimney-tops depends ; it is 

 also important in its bearing on the position of the mouths of air-trunks 

 for hot-air furnaces ; if the mouth be placed in a point of diminished 

 pressure, on the leeward side of a building, air may pass outward, 

 especially from apartments on the windward side of. the house. 



"4. When a current strikes the extremity of a tube perpendicularly 

 to its axis, motion is produced through the tube towards the current ; 

 and when a current already exists in the tube, if its velocity is less 

 than that of the impinging current, that velocity will be increased, 



" When two currents of air of different velocities are moving in pre- 

 cisely the same direction, the influence of the more rapid current in 

 accelerating that which is less rapid is not so great as when the angle 

 of meeting is between 20° and 40°. When two opposite currents of 

 equal diameter and velocity meet, they form a circular sheet, perpen- 

 dicular to the axis of the veins, and the resulting phenomena resem- 

 ble those arising when a current strikes a circular plane. If the ve- 



* A simple demonstration of these propositions may be obtained by means of a 

 card and candle. If a blast from the mouth be directed obliquely against a card, 

 the flame of a lighted candle will be drawn towards the card, on whatever side 

 of it the candle is held. Increasing or diminishing the velocity of the blast does 

 not change the direction assumed by the flame, but only the velocity with which 

 it is drawn towards the card. 



If the blast be directed perpendicularly upon the centre of the card, the flame, 

 when passed around the edge of the card, will be driven outward at all points ; 

 and if the candle be held near the blast, and at a little distance from the plane 

 surface, the flame will, in virtue of the lateral communication of motion, be drawn 

 towards the surface, and yet by the current of air close to and parallel with the 

 card it will be prevented from reaching it. A strong flame may thus be made to 

 play, apparently with great force, upon the hand, and yet not burn it. An illus- 

 tration of this principle may often be observed in the narrow pathway, so con- 

 venient for foot-passengers, found after a snow-storm, on the windward side of a 

 high and close fence. 



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