172. =Syphon.=--The pressure of air upon fluids is beautifully exemplified in the operation of the syphon. This instrument is simply a bent tube having one branch longer than the other. Its operation is shown in Fig. 117. The tube having been first filled with the liquid, has its shorter branch in the liquid of the vessel A, which is to be emptied, and the other in the vessel B, which is to receive the liquid. As you see it here, the opening of the long branch is below the surface of the liquid in B. It is manifest, therefore, that the air presses equally upon the surfaces in both vessels, tending to support the fluid in the tube, just as the fluid is supported in the jar in Fig. 114. But, notwithstanding these equal pressures, the liquid runs up the tube from A, and down its longer branch into B. Why is this? As the pressure of a column of fluid is as its height, there is greater pressure or weight in the longer branch than in the other; and it is this difference in weight that causes the flow from A into B through the syphon. The difference in the columns in the two branches is not the difference in length of these branches, but the distance between the levels of the fluid in A and B, that is, the distance from _a_ to _b_. The operation, then, of the instrument is this. There is a constant tendency to a vacuum at C, the bend of the tube, from the influence of gravitation on the excess of fluid in the long branch over that in the short one. This tendency is constantly counteracted by the rise of fluid in the short branch, it being forced up by the pressure of the air upon the surface of the fluid in A. [Illustration: Fig. 118.] If the syphon were so placed that the surface of the liquid in A is precisely on a level with that in B, as represented in Fig. 118, the liquid would remain at rest, for as pressure is as the height, § 121, and the pressures on the two surfaces are equal, there would be an exact balance. But let the surface in B be in the least lower than in A and the flow will begin. And the greater the distance between the two levels the more rapid will be the flow, for the greater will be the influence of gravitation in the long branch. [Illustration: Fig. 119.] Again, if the end of the long branch of the syphon be free, as in Fig. 119 (p. 130), the syphon will operate in the same way, for the air, pressing in all directions equally, tends to support the column of fluid in the long branch by a direct upward pressure, but is prevented from doing so by the excess of fluid in it above what is in the shorter one. The operation of the syphon is commonly represented in this way; but I have given first the arrangement in Fig. 117, in order that you might more clearly see the principle of the instrument. [Illustration: Fig. 120.]