WAVES. Undulations of fluids pro duced by displacements of the particles at some distance, and the subsecrent ef fort of these to regain their equilibrium, or place themselves upon the same fluid level. The waves of the ocean produced by the action of the winds never attain , the height which it is commonly esti mated they do. Dr. Scoresby, in his estimation of the waves of the Atlantic Ocean, found the average wave to be 15 feet above the level of the water, suppos ing it to be a smooth plane ,• and the mean highest waves, not including the broken or acuminated crests, to be 43 feet above the hollow, or trough of the sea, produced by the walling up of the wave. It is only the upper stratum of the water which is thus agitated (from 15 to 20 feet from the surface), and the wave is not carried forward, but rises up. and down, and thus displaces a fresh body of water in advance, compelling it to rise up, and become a wave in turn.
There are waves in the atmosphere as well as in the ocean, and the upper re gions of the air have their currents, tides, and waves, just as the oceans of water. It is the elevation and depres sion of these groat atmospheric waves which produce a corresponding move ment in the mercury of the barometer tube.
This physical phenomenon of undula tion is not confined to fluids and gases : it appears to be a property of tho ether which fills space, agitations of which produce such interesting effects. Heat and electricity, it is presumed. are par ticular forms of undulation of the ether, and light is now generally admitted to be waves of ether ; the nature and color of the light being produced by varieties in the undulation. To produce an ordi nary ray of white light, it is not merely necessary that the ether should be agi tated by horizontal waves, like those of the ocean, but that it should also have waves vibrating vertically, or from side to side : and these two kinds of waves are bound together in the ordinary ray of sunlight. These two rays may be separated by passing the sunlight through certain substances, as a clear crystal of Iceland spar, doubly refracting spar, a variety of carbonate of lime. By turning the crystal round in the hand above any object, such as a single letter on a white surface, two images of this letter will be perceived, one of which remains station ary, or nearly so, while the other travels round it in a circle ; the latter is called the extraordinary, the former the ordi nary ray. The phenomenon is produced by the refractive power of the spar, which thus resolves the luminous undu lations into two colorless series at right angles to each other ; when combined, constituting common light, and when separated, producing polarized light, so called, because they assume new and pe culiar properties with to each other, and different refracting or reflect ing media.
Many minerals polarize light, as agates and tourmalines. Light thus altered may also be obtained by reflection from surfaces, such as glass ; and it was from this source it was first discovered by Mains. By more complicated methods, the ray of colorless light may be so much refracted as to be broken up into its colored rays, and then acting in a ner similar to the colorless ray, produces the phenomenon of colored polarization. This subject cannot be entered into nutely in this treatise. Polarized light is now made use of in the arts as a test for the purity of substances. It is found that liquids lave the property of ing light similar to crystals, and not only is the amount of separation of the two rays constant at all times for the same substance, but even the direction is also permanent and unvarying ; thus some substances turn the polarized ray to the right, and others to the left. It is only necessary, therefore, to know what the amount and direction of the deviation of the ray is, and then by looking into the tables made for that purpose, the act substance will be found opposite to the figures in the table. Solutions of sugar, camphor, and many other organic liquids, naturally develop the enon of circular polarization,—that is, make the ray revolve round. Oil of pentine makes the ray rotate to the right ; so also does naphtha and oil of anise, syrup of grape sugar and grape juice; while oils of citron and mot, solution of cane sugar, and tartaric acid, make the ray rotate to the left. Polarization is often made a useful test of the presence and purity of essential oils and volatile liquids. The instrument with which these experiments are made is called a scope; an tration of which is given. It sists of a tube of brass one inch broad, and eight inches long, A, B, into which the fluid to be examined is placed. The tube is dosed at the lower end by a plate of glass. At one extremity of this tube is placed a bundle of plates of dow-glass IV fixed so as to admit of ready motion, and supported by a screw in Its place. These plates of glass receive the ray of light, polarize it, and transmit it through the tube containing the liquid. At the other end of the tube is placed the eye-piece M, consisting of a single image prism, or Nicholl's prism, which is a crystal of refracting spar sawed down the middle, and soldered together by Canada balsam. A bundle of thin glass plates may be used instead of this, if they are capable of being placed at any azimuth.