[The old “catch” question of the “difference in weight between a pound of lead and a pound of feathers” here comes to the mind. The answer generally given is that “feathers make the heavier ‘pound’ because they are weighed by avoirdupois, and lead by troy weight.” This is an error. They are both weighed in the same way, and pound for pound are the same _weight_, though different in _volume_.] Fluids in equilibrium have all their particles at the same distance from the centre of the earth, and although within small distances liquids appear perfectly level (in a direct line), they must, as the sea does, conform to the shape of the earth, though in small levels the space is too limited to admit of any deviation from the plane at right angle to the direction of gravity. Liquids always fall to a perfectly level surface, and water will seek to find its original level, whether it be in one side of a bent tube, in a watering pot and its spout, or as a fountain. The surface of the water will be on the same level in the arms of a bent tube, and the fountain will rise to a height corresponding with the elevation of the parent spring whence it issues. The waterworks companies first pump the water to a high reservoir, and then it rises equally high in our high-level cisterns. As an example of the force of water, a pretty little experiment may be easily tried, and, as many of our readers have seen in a shop in the Strand in London, it always is attractive. A good-sized glass shade should be procured and placed over a water tap and basin, as per the illustration herewith. Within the glass put a number of balls of cork or other light material. Let a stop-cock, with a small aperture, be fixed upon the tube leading into the glass. Another tube to carry away the water should, of course, be provided, but it may be used over again. When the tap is properly fixed, if the pressure of the water be sufficient, it will rush out with some force, and catching the balls as they fall to the bottom of the glass shade bear them up as a juggler would throw oranges from hand to hand. If coloured balls be used the effect may be enhanced, and much variety imparted to the experiment, which is very easy to make. [Illustration: Fig. 57.—Water jet and balls.]