NoVKMUEK 10, IS^S.I 



SCIENCE, 



639 



A man reiuls, or sees in a public hall, tliat 

 two oleeti-ltied pith-balls attract or rei)cl each 

 other. He learns that the hiunan bod}- niaj- 

 be charged with electricity. Straightway he 

 begins, upon this basis, to explain the table- 

 tipping feats of spiritualistic mediums, — a 

 gross error, hardly more respectable than the 

 pure superstitition of the veriest believer in 

 ghosts. 



To make such errors impossible would re- 

 quire that definite, familiar knowledge of things, 

 in their quantitative relations, which is hardly 

 to be obtained without actual contact. It 

 would require a laboratory training ; and it is 

 perhaps impossible to make provision for a 

 very extended training of this sort in any 

 scheme of general education. 



The tendency of the times, however, is 

 toward the <)l>jective and experimental in teaclir 

 ing ; and it is probable that the next few years 

 will see considerable changes in the methods 

 of general instruction in physics. 



WHIKLWINDS, CYCLONES, .lyi) TOIt- 

 NADOES.^ — Ul. 



We may now pass on from the small day- 

 time w hirls of dry air to the larger, long-endur- 

 ing storms that are accompanied by ruin ; and 

 here will be met two new elements, — the etToct 

 of condensing vapor, and the eirect of the 

 earth's rotation, — both of great importance. 

 As a sample under this second lieading, we 

 may take one of the cyclones of the Bay of 

 Bengal ; for the storms there are very charac- 

 teristic of their class, and have of late years 

 received much careful attention. There is 

 good reason for thinking that these cyclones 

 generally s|)rrng up in calms, much as the des- 

 ert-whirls begin. The seasbns and regions of 

 their occurrenee l)oth |)oiut to that conclusion ; 

 for tr(J^>ical cyclones seem never to liegin in 

 well-established wind-currents, hut rather in a 

 place of quiet, weak, or variable winds. By 

 India, for example, the cyclones ar» almost 

 unk^ow^l during the prevalence of the steady 

 blowing monsoons, but are not uncommon at 

 those seasons when the monsoons change : that 

 is, at times when the air has no well-established 

 motion, but stands about idly, waiting for a 

 decisive command to move on. During these 

 idle times of stagnation, the lower air m.iy 



I (ODtlDDeil from No. 4(l. 



become very warm and moist, and so pre|)are 

 for a stormy overturning. The calm that pro- 

 cedes a cyclone often makes part of the de- 

 scription of a storm at sea : the air is close 

 and opi)ressivcly warm ; the water settles down 

 to a glassy surface ; and now we ma^- see, 

 what is not always clearly expressed, that this 

 calmness of the water, and oppressive heat of 

 the air, are not antecedent effects of the com- 

 ing storm, but are actually tiie conditions that 

 allow and determine the beginning of a storm. 

 The warmer the air and the quieter the water, 

 the longer must have been the preparatory 

 stage : the greater the quantity of solar force 

 collected in tiie lower atmosphere, the more 

 violent will l)c tlie storm when it begins. This 

 warm cahn is really the embryo of the cyclone ; 

 and, if it lie long euongh iu a proper latitude, it 

 will grow to well-developed maturity. 



It is often stated that tropical oceanic cy- 

 clones begin at the meeting of two opposite 

 currents of air rather than at a time of calm. 

 This may be true for some cases, and undoubt- 

 edly has a very general application in temper- 

 ate latitudes ; but it seems more probable that 

 in the Bengal cyclones, and most other tropical 

 hurricanes, this stage is a little later than the 

 earliest beginning, atid is really the first de- 

 velopment of the inblowing winds. A general 

 calm would doubtless be found to precede such 

 opposed cuiTents if observation could trace 

 the antecedent conditions a little farther back 

 than is usualh' possible. The principal con- 

 trasts between the desert-whirls and the Bengal 

 cyclones, at tlie time of their beginning, may 

 be thus summarized : — 



First, The «rca and uniformity of the sur- 

 face on which the disturbance is developed is 

 much greater on the ocean than on the desert. 



Second, There is a. lower temperature, but a 

 much greater amount of heat, surface for sur- 

 face, in the cyclone's embryo, than in the whirl- 

 wind's. The temperature of the air over the 

 oceaa seldom exceeds !).5° : over the desert 

 sands it may often rise to 140° or 150° close 

 to the ground. But on the desert the stratum 

 of air tiiat is so excessively warmed is very 

 thin ; it oft* fails to reach the height of a 

 man's e3-e, and so gives the apfiearance of 

 a mirage : while over the sea, althougli the 

 lower stratum is not so warm, its thickness is 

 greater, and there is more of it warmed. What 

 it lacks in temperature it more than makes up 

 in quantity. 



Third, The presence of water-vapor over the 

 ocean makes a most important contrast between 

 the two cases; aud it is <m this account that 

 the warm sea-air is cooler than the hot desert- 



