612 



SCIENCE. 



[Vol. II., No. 40. 



Manifestly it must stop in the afternoon, as the 

 sun's heat decreases ; and it can never occur 

 at night, for then the surface-air is, as a rule, 

 cooler than that above, and the atmospheric 

 equilibrium is correspondinglj' stable. Further, 

 the whirl will remain at one place, unless, as is 

 often the case, it is carried along bj' a general 

 motion of the upper air. 



There is a A'erj- strong point of evidence, if 

 any is needed, in favor of the view that heat 

 applied to the lower layers of the air will pro- 

 duce a whirlwind. This is the fact of their pro- 



FlG. 4. (Taken from Abhandl. gesellsch. 



Gott.) 



duction over fires. Much interest was excited 

 in this question in connection with the arti- 

 ficial causing of rain, some forty years ago, in 

 this countrj' ; and observations were carefully' 

 made of the whirls formed over liuruing woods 

 and canebvakes, showing them to be ver^- sim- 

 ilar in form and action to those naturallj- aris- 

 ing on drj- plains (fig. 3) . Similar whirls have 

 been seen over volcanoes (fig. 4) ; and on a 

 calm day the smoke ascending from a factor3r 

 chimnej- may be seen to have a slow rotary 

 motion. Heat is therefore an amply sufficient 

 cause of such disturbances. No other excite- 

 ment is needed, and electricitj' has no essential 

 part to plaj-. In recognizing this, we see the 

 chief difference between the older and newer 

 theories of storms. 



Sand-whirls are common in all desert or 

 dr^' I'egions, where the}' often have the name 

 of spirits or devils, from the fantastic and 

 apparentl}' evil way in which the}' flit across 

 the burning sands. They have neither clouds 

 uor rain. When well and frequentl}' devel- 

 oped, they ma}' grow to dangerous strength, 

 and lift much dust and sand into the upper air, 

 where it is blown long distances before falling. 

 In this way they serve as important geologic 

 agents. Vessels west of the Sahara, or east of 

 China, are tlms often powdered over with fine 

 dust slowly settling down after a long flight 

 from its desert source. 



The smaller water-spouts, doubtless, belong 

 near here in our scheme of classification ; but 

 as they are usually aided by vapor-force, and 

 approach the character of tornadoes, their con- 

 sideration is best deferred till later. 



Finally, before going on to the larger storms, 

 one point of much importance must be empha- 

 sized. The change from the stable equilibrium 

 of night and early morning to the unstable of 

 noon is eff'ected entirely by the sun's heat, 

 which warms the lower air, and causes it to ex- 

 pand. In expanding, it lifts all tlio upper air 

 that rests on it ; and this is no small piece of 

 work, for the air that is lifted weighs about a 

 ton over every square foot. When a point of 

 escape is found, the heavy upper air sinks 

 again, as the expanded air is drained oif (up- 

 wards) at the centre. It is this gra\'itative 

 force of the sinking air-mass that causes the 

 dust-whirlwind, in re-arranging the disturbed 

 equilibrium of the atmosphere ; but gravity 

 would have no chance to show its strength, if 

 the air had not been lifted by force fi'om the 

 sun. The winds of a dust-storm, therefore, 

 depend on gra\-itative force brought into play 

 by the sun's heat. All storms and all winds 

 have more or less closely this relation to solar 

 energy and terrestrial gravity. 



{To be continued.) 



THE INTERNATIONAL FISHERIES EX- 

 HIBITION. — FO UR TH PA PER. 



On the 1st of October, at noon, the number 

 of visitors to the exhibition passed the much 

 desired limit of two millions ; and, although 

 the rainy season had set in, the daily average 

 of attendance was still increasing. The finan- 

 cial success of the enterprise was more than 

 certain two months ago ; and the receipts of 

 each day since have been swelling the surplus 

 fund, the disposal of which is now a fruitful 

 subject of discussion in England. Although 

 the organization is a private one, the character 



