ON HYDRAULIC PRESSURil. S05 



its surface, there will still be an elevation before and a depression behind it, 

 the less in height, and the greater in extent, as the depth at which the body 

 is situated is greater. Such an elevation appears to be in some measure 

 analogous to the effect of a low were thrown across a river, which raises its 

 surface, and produces a swell. 



If two or more bodies, differently formed, the resistances to the motions of 

 which had been ascertained, were caused to move through a fluid in contact 

 with each other, it is obvious that the paths described by the particles of the 

 fluid, in gliding by them, must be very materially altered by their junction; 

 and it seems natural to expect that the joint disturbance produced in the 

 motions of the fluid, when the surfaces are so united as to form a convex 

 outline, would be somewhat less than if each surface were considered sepa- 

 rately. Accordingly it is found that no calculation, deduced from experiments 

 on the resistance opposed to oblique plane surfaces, will determine with ac- 

 curacy the resistance to a curved surface. It appears from experiment that 

 the resistance to the motion of a sphere is usually about two fifths of the re- 

 sistance to a flat circular substance ©f an equal diameter. The resistance to 

 the motion of a concave surface is greater than to a plane, and it is easily 

 understood, that since the direction, in vi^hich the particles of the fluid recede 

 from the solid, must be materially influenced by the form of the solid exposed 

 to their action, their motion in this case must be partly retrograde, when- 

 they glide along towards the edges of the concave surface, and a greater 

 portion of force must have been employed, than when they escape with a small- 

 er deviation from their original direction. (Plate XXI. Fig. 276.) 



For some reason which is not well understood, the hydraulic pressure of 

 the air appears to be somewhat greater, in proportion to its density, than that 

 of water. It has been found that the perpendicular impulse of the air, 

 on a plane surface, is more than equivalent to the weight of a column of air 

 of a height corresponding to the velocity, and the excess is said by some to 

 amount to one third, by others to two thirds of that weight. The resist- 

 ance appears also to be a little greater for a large surface, than for a number of 

 smaller ones, which are together of equal extent. 



The resistance or impulse of the air, on. each square foot of a surface directly 



VOL. T. B r 



