ADHESION OP DISKS. 175 



The extent of the valve, we will suppose to equal four square 

 inches, its surface flat, and placed on the steam chest, where the 

 surface is also flat. We will also suppose a space between the sur- 

 faces, sufticient to admit the atmosphere, it is evident that the top 

 and bottom of the valve will be then equally pressed by it, and ia 

 that state the valve may be lifted with a force only sufticient to 

 overcome its gravity. 



Suppose an opening to be now made in the chest, under the 

 centre of the valve, equal to one square inch, or one-fourth the 

 size of the valve, the steam being then let into the chest with a 

 power of 30 pounds to the inch, this rushes against, and communi- 

 cates the same power to the one inch of the valve immediately 

 over the aperture, which being only about half the force of the at- 

 mosphere on the whole four inches on the top of the valve, it can- 

 not rise with any thing like the velocity with which the steam 

 would move ; its current, therefore, must necessarily be checked. 

 Now currents of fluids on meeting immoveable obstacles, are not 

 reflected back, like solids, but take a direction parallel to the sur- 

 face against which they strike : currents of fluids also, whether 

 elastic or nonelastic, exert no force, but in the direction in which 

 they move ; the latter is fully proved by forcing air, or water, 

 through a cylindrical tube, if holes be made in the sides of the tube 

 none of the fluid will escape. 



It follows, then, that the steam on striking against the valve, as 

 above mentioned, and thus arrested in its course, which was per- 

 pendicular to the valve, takes one parallel to it, and rushes out in 

 all directions, in radii from the centre ; of course, the atmosphere 

 between the disks is driven out, leaving nothing between but steam, 

 and that exerting no force on the valve except on the one inch in 

 the centre, its course over the remaining surface is parallel to it, 

 and its force is exerted only in that drection. 



The state of the valve would then be as follows : pressed up- 

 wards by steam of 30 pounds to the inch, acting on the one inch 

 only at the bottom, and pressed downward by the weight of the at- 

 mosphere of 15 pounds to the inch, acting on the whole four inches 

 at top ; making the downward, double that of the upward pressure. 



The above will appear pretty evident, by the following experi- 

 ment : take two thin flat plates of metal, tin-plate for example, 

 about three inches diameter, in the centre of one insert a tube 

 several inches long, and about l-8th of an inch in diameter per- 

 pendicular to ith planes ; let both plates be pierced full of holes 

 except near their centres ; let these plates be placed with their 

 surfaces parallel to each other, and about 1-1 6th of an inch apart, 

 and attach the tube to a cock under a considerable head of water •, 

 when the water is let on it passes by the tube through the first 

 plate, and on being checked by the second plate, passes out be- 

 tween them in a thin and even sheet, while none escapes through 

 the holes. This happens when the force of water is at least equal 

 to two atmospheres. This experiment has been repeatedly tried; 

 and varied in diflerent wavs. with the same uniform result. 



