NOVEMBKK 30, 1885.] 



SCIENCE. 



im 



queiilly. Cor evi'iy throe hundred feel of ascent 

 it has an advantage over dr}- air of one degree 

 of warmtii (and more at great altitudes), tend- 

 ing to make it lighter than its surroundings. 

 and so intensifying its upward motion. ^lore- 

 over, a storm wliieh is thus nourished may 

 continue its activity through the niglit. instead 

 of dying away as the sun declines ; for it is 

 supplied with energy continually brought out 

 of the vapor storehouse. Of course, in both 

 cases the sun's heat is the source of the dis- 

 turbance ; hut on the desert there is no way 

 of storing up the heat, while at sea a great 

 amount of energy may be stored up before the 

 final upsetting begins, and 

 then the storm-winds arise, 

 and show all this accunnilat- 

 ed strength in their l)lowing. 



We have much this kind of 

 action, ii» a small way, in the 

 formation of a heavy cumulus- 

 cloud on a quiet, hot summer 

 day. The air on the ground 

 is warmed, and contains a 

 good share of moisture ; and. 

 as it rises and cools, its vapor 

 begins to be condensed, .'^ouio of the vapor- 

 energy is given out as heat, and so the ascend- 

 ing current is re-enforced. If the air be verv 

 warm or very moist, or both, the ordinary 

 cumulus-cloud may grow into a thunder-shower ; 

 and, being tiien unable to carry up all its con- 

 densed vapor, some of it falls as rain. It 

 should be noted, that, when the lower air is not 

 fully saturated, its temperature must he some- 

 what reduced to bring it to tlie point of satura- 

 tion before any cloud is formed. This decrease 

 is mechanically effected at the rate of 1 X>° every 

 three hundred feet, by the expansion of the 

 rising air, — essentially the same rate as that 

 already given for the cooling of a rising column 

 of dry air ; and, when enough cooling has been 

 thus effected to reduce the air to its tempera- 

 ture of saturation, some of the vapor will be 

 condensed into liquid cloud-particles, and so 

 become visible. It is for this reason that 

 cumulus-clouds have nearly level bases, and 

 that a groni) of such clouds stands at about the 

 same altitude. The air-currents rising from the 

 warm ground have to ascend a certain distance, 

 and cool a certain number of degrees, before 

 condensation takes place. Their altitude in feet 

 will be about a liundrcd and eighty-three times 

 the number of degrees between the temperature 

 of the lower air and its dew-point. 



All tropical cyclones arc attendeil by clouds 

 and by excessivch" heavy rain ; and this points 

 very clearly to the important [lart played by 



the heat evolved in the condensation of so much 

 vapor. The rapid reproduction of tlic heat 

 storiMl up through many j)rcvious d.iys of sun- 

 shine retards the cooling of the ascending cur- 

 rent, excites the winds to active motion, and 

 the storm is thus set going. Kspy (1^3.')) was 

 thfi first to recognize the in)portant part played 

 by the condensing vapor in an ascending cur- 

 rent of air, but he greatly exaggerated its 

 effects. The proper measure of its action, and 

 convenient statement of the results in tabular 

 form, are chieflv due to Reve (18(!4) and Ilann 

 (1874). 

 The ascending current moves outward at a 



height of one or two miles, spreading itself 

 over the surrounding atmosphere. To show its 

 relation to the storm circulation, we may refer 

 to the following figures. Fig. (1 shows the air 

 in a (juiescent state, before the storm begins. 

 .^t such a time, there being no wind, the weight 

 of the air. or the barometric pressure at sea- 

 level. — say, 3<i inches, — is uniform throughout 

 the area preparing for cyclonic disturbance. 

 The pressure is uniform, not only at the sea- 

 surface, but also at any given altitude above it 

 (the effect of the upper winds is here omitted 

 as being non-essential to the explanation, as 

 well as unknown) ; so that the lines in the fig- 

 ure will represent level surfiices of equal press- 

 ure of 28, 26, 24 inches, or isobaric planes 

 at altitudes of about l.iiOO, ;1,:jOO, and .■),000 

 feet. As long as the vertical gravitativc press- 

 ure is at right angles to these planes, the air 

 is not tempted to move, but will remain at rest 

 till disturbed by some new condition. This 

 new condition will be some form of the disturb- 

 ing actions already suggested, by which a cen- 

 tral region of greatest warmth is detei-mined, 

 in consequence of which there will be an ex- 

 pansion of the atmosphere at that place. The 

 isobaric planes will become convex there, as in 

 fig. 7 ; for the altitudes at which barometric 

 pressures of 2.s, 2(i, 24 inches arc found must 

 now be greater than before. As there has 

 t)eeD, as yet, no lateral motion, this produces 

 no change in the pressure at sea-level. But a 



