1S2 OUTLINES OF NATURAL PHILOSOPHY .

concavity of its surface that makes the water inthe tube press less on the bottom than if its surfacewere plain, but it is the attraction of the glass thatproduces, in a manner equally direct, both the con-cavity and the diminution of the pressure.

276. The reason why the water between thetwo narrow plates of glass rises only to half theheight it does in a capillary tube, of a diameterequal to the distance of the plates, is, that thequantity of the attracting zone is nearly halved.

The same holds of the tubes, inserted the one withinthe other. Any small column of the water be-tween them, is supported by the attraction of theglass only on two sides.

So also, in the movement of the drop to the smallend of a conical capillary tube; if the attraction ofa particle of the tube be resolved into two, one inthe direction of the axis of the tube, and the otherat right angles to it, the former will be directed to-wards the vertex of the cone. If the force is repul-sive, the part in the direction of the axis will betoward the base. On the same principle may be ex-plained the tendency of two plates to unite, whe-ther water rises or mercury sinks between them.

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