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HYDROSTATICS AND PNEUMATICS.

(Cabinet Cyclopaedia, vol. xvii.)

This volume is in every respect worthy of standing beside the luminous Discourse on the Study of Natural Philosophy, by Mr. Herschel.  It is just in the method that we wish to see all branches of science treated, and it is the only means of rendering such knowledge familiar; and this has only to be known to become popular.  We understood this to be the aim of the Cabinet Cyclopaedia at its outset, and the scientific volumes already published are an earnest of the Editor’s zeal and success.  The best method of illustrating this recommendation, is to seize from the volume a few familiar effects whose causes are imperfectly understood, and thus to show how closely the spread of science is identified with civilization and the common comforts of social life:—­

Deceptive appearance of Waves

If we observe the waves continually approaching the shore, we must be convinced that this apparent motion is not one in which the water has any share:  for were it so, the waters of the sea would soon be heaped upon the shores, and would inundate the adjacent country; but so far from the waters partaking of the apparent motion of the waves in approaching the shore, this motion of the waves continues, even when the waters are retiring.  If we observe a flat strand when the tide is ebbing, we shall still find the waves moving towards the shore.

Ornamental Fountain Clocks.

It is the same cause (that which produces the deceptive appearance of a progressive motion in the waves of the sea) which makes a revolving cork-screw, held in a fixed position, seem to be advancing in that direction in which it would actually advance if the worm were passing through a cork.  That point which is nearest to the eye, and which corresponds to the crest of the wave in the former example, continually occupies a different point of the worm, and continually advances towards its extremity.—­This property has lately been prettily applied in ornamental clocks.  A piece of glass, twisted so that its surface acquires a ridge in the form of a screw, is inserted in the mouth of some figure designed to represent a fountain.  One end of the glass is attached to the axle of a wheel, which the clock-work keeps in a state of constant rotation, and the other end is concealed in a vessel, designed to represent a reservoir or basin.  The continual rotation of the twisted glass produces the appearance of a progressive motion, as already explained, and a stream of water continually appears to flow from the fountain into the basin.

Facility of Swimming.

The lighter the body is in relation to its magnitude, the more easily will it float, and a greater proportion of the head will remain above the surface.  As the weight of the human body does not always bear the same proportion to its bulk, the skill of the swimmer is not always to be estimated by his success; some of the constituent parts of the human body are heavier, while others are lighter, bulk for bulk, than water.  Those persons in whom the quantity of the latter bear a greater proportion to the former, will swim with a proportionate facility.