The causes of wind augment or diminish the gravity or elasticity of the atmosphere; for two portions of air, which are equal in elasticity or gravity, remain mutually immoveable. We must look for the causes of wind in the variation of heat and cold, the position of the sun, the nature of the soil, the inflammation of meteors, the condensation of the vapors into rain, and other similar circumstances: but the most general causes are heat and cold. Fire, which expands and rarefies the air, diminishes its elasticity, and, consequently, makes it lighter in some places than in others; hence the pressure of the ambient air is greater than that of the rarefied, whence a motion arises; and thus several winds blow towards the part where the air is rarefied by the heat; which currents of air, if strong, are called _winds_, if gentle, _breezes_ or _gales_. Thus the air is constantly carried from the polar regions towards the torrid zone, where it is also affected by the diurnal motion of the sun from east to west. “When we reflect attentively upon the nature of winds in general,” says Dr. O. Gregory, “considering all the causes which disturb the equilibrium of the atmosphere, the great mobility due to its fluidity and its elasticity, the influence of heat and cold upon the latter, the immense quantity of vapor with which it is charged and discharged alternately, the mutual effect of contiguous air and water in motion, the varied attractions of the sun and moon, upon the aërial fluid, and finally the changes produced by the earth’s rotation in the velocity of the atmospherical moleculæ at different parallels of latitude; we shall no longer be astonished at the inconstancy and variety which infringe upon the regularity of some of our winds, nor of the extreme difficulty of reducing the whole to laws wearing the semblance of certainty.”[66] There is a great variety of winds. The ancients observed only four, called _venti cardinales_, because they blow from the four cardinal points. Homer mentions no more than _eurus_, the east; _notus_, the south; _zephyrus_, the west; and _boreas_, the north wind.[67] In imitation of him, others do the same. Afterwards intermediate winds were added, first one, then two, between each of these. Most writers, make only eight winds, and Vitruvius[68] informs us that the Athenians built a marble tower in the form of an octagon with eight winds marked, every one on that side which faced it. The moderns make 32 winds, the four cardinal winds 90 degrees distant, and 28 collateral or intermediate, 11 degrees and 15 minutes distant from each other, of which those in the middle between two cardinals, are 45 degrees distant from each cardinal.[69] But some make as many points on the compass, and as many winds, as there are degrees on the horizon, namely, 360.