1098

THEORY AND PRACTICE OF ENGINEERING.

Book II.

Having a vessel, A B C D, filled with any fluid to the level I K, let us imagine that inthis vessel we form with the same liquid a glass siphon GE.MNfHQP G, serving as adiaphragm, it is evident that this liquid, which the siphon would contain, should be in thesame state as it was before its separation from what remains in the vessel; but as, before theformation of the siphon, the parts of the liquid which it contains were in equilibrio, andthe surfaces G E and FII level, the same things would still subsist.

a n

A I)

Fig. 1661.

The liquid contained in the siphon having nothing in common with what remains in thevessel, if we suppose the latter annihilated, and the glass siphon transformed into someother material, as copper or tin, we shall see that water poured into a siphon of any figurewill assume its level in the two branches, and maintain its equilibrium, however unequalthe size of these branches may he : notwithstanding this law of nature, there are casesin which liquids do not rise to their level, as will be noticed hereafter.

If we have a tube open at both ends, and weplunge a part perpendicularly into the water, itenters therein and maintains the level of thesurface of the water, since we may regard it asa column which is in equilibrio with thatwithout, in the same manner as it was previousto its inclosure in the tube ; but what is sur-prising, this does not happen in all tubes; andin capillary tubes, or those of very small dia-meter, the water mounts above the level of thatwhich is outside, and the higher in proportionas the diameter is less.

All columns of water tend by their weight to descend and raise each other, and it is theequality of their force which keeps them level : if it should happen that one of thesecolumns were lighter than the others it would immediately rise above the others to theheight necessary for the equilibrium. When a capillary tube is placed on the surface ofthe water, the drops of which are comprised in its opening attach themselves to the interiorof the small circle which forms it, are sustained bv it in part, and consequently are somuch the lighter with regard to the water, which acts freely on the bottom of the vessel.Then the column of water, which answers to the opening of the capillary tube, exercises itsweight less on the bottom of the vessel than the other columns, with which it is surrounded,so these latter ought to elevate it in the capillary tube to a height at which it regains by agreater quantity of water the weight by which it is deficient with regard to the bottom ofthe vessel, to be in part sustained by the drops of water which adhere to the tube. Itfollows that the smaller the diameter of the tube is, the more the water within wouldelevate itself above the level of the other, as having more surface in proportion as a greaternumber of drops of water are sustained by it; and those of the middle will have so manythe more leaning points in the part of the preceding, as the tube is narrow.

Another experiment which appears coyitrary to liquids always keeping in equilibrio. —Having a vessel A B in which water or any other liquid is placed to any height C D, if wetake a piece of cloth and dip it in the water, then placingit on the edge of the vessel so that the part G H dips in thewater, and the other E F is outside the vessel, and sufficientlylong for its extremity E to he below the surface C D of theliquid, we shall see it mount along the band, drop from theextremity E, and the vessel will be entirely emptied if theextremity II reaches the bottom.

Gardeners adopt this method to water plants which requireto be kept moist, and chemists apply it in a way whichfrom its singularity requires mentioning. When thev haveseveral liquids mixed which they desire to separate, they dipcloth bands into each of the pure liquids; they then dis-pose these bands on the edge of the vessel, and each distils theliquid with which it was wetted, as well in a vacuum as in a plenum.

A D

Fig. 1665.

Fig. 1666.