Were it not for this reflection, one might endeavour to show how all the great Easterly trade winds--the no less important anti-trades,[29] or nearly constant Westerly winds,--and their complicated eddying offsets, are all (on greater or smaller scales) breadths, or zones of atmosphere, alternating, or circulating, or crossing (superposed or laterally)--between which, at distant intervals, occur those strong eddies, or storms, called hurricanes--typhoons--tornadoes--or cyclones. The great easterly and westerly movements--so clearly shown by philosophers to be the consequences of cold polar currents of air--warm equatorial currents--and diurnal rotation of the earth;[30] are grand ruling phenomena of meteorology--to which storms, and all local changes, occurring but occasionally, are subordinate and exceptional. Further investigations into electrical and chemical peculiarities will probably throw additional light, perhaps the strongest, on meteorological science. * * * * * In the previous observations, general reference has been made to mercurial barometers of the ordinary kind; but, excepting the construction of the instruments themselves, those observations apply to all barometers, wheel--aneroid--or metallic--and likewise, of course, to the sympiesometer, which is a modified barometer. But as these four last-mentioned instruments are scarcely so familiar as the simplest form of barometer, it may be useful to add a few words about each of them. * * * The WHEEL barometer has a syphon tube, partly filled with mercury, on which, at the short or open end of the tube, a float moves, to which a line is attached that moves a wheel, carrying an index.[31] * * * ANEROID barometers, if often compared with good mercurial columns, are similar in their indications, and valuable; but it must be remembered that they are not independent instruments; that they are set originally by a barometer,[32] require adjustment occasionally, and may deteriorate in time, though slowly. The aneroid is quick in showing the variation of atmospheric pressure, and to the navigator who knows the difficulty, at times, of using barometers, this instrument is a great boon, for it can be placed anywhere, quite out of harm's way, and is not affected by the ship's motion, although faithfully giving indication of increased or diminished pressure of air.[33] In ascending or descending elevations, the hand of the aneroid may be seen to move (like the hand of a watch), showing the height above the level of the sea, or the difference of level between places of comparison.[34]