436 



NATURE 



[December 2, 1920 



Electric Switch and Controlling Gear: A Hand- 

 hook on the Design, Manufacture, and Use of 

 Switchgear and Switchboards in Central 

 Stations, Factories, and Mines. By Dr. C. C. 

 Garrard. Second edition, revised and enlarged. 

 Pp. xxii + 654. (London: Benn Brothers, Ltd., 

 1920.) Price 25s. net. 

 No considerable alterations have been made in 

 this work since the first edition was reviewed in 

 Nature of March i, 191 7- Slight modifications 

 have been effected, and recent data in connection 

 with high-tension gear, lightning arresters, etc., 

 added. Two new sections, one dealing with the 

 standardisation of switchgear and the other with 

 automatic contactor switches, have also been in- 

 serted. 



Milk Testing: A Simple Practical Handbook for 

 Dairy Farmers, Estate Agents, Creamery 

 Managers, MUk Distributors, and Consumers. 

 By C. W. WaJker-Tisdale. Second revised 

 edition. Pp. 90. (London : J. North, Dairy 

 World Office, 1920.) Price 35. 6d. net. 

 The recognition of the value and importance of 

 "milk recording" is making it increasingly neces- 

 sary that simple but trustworthy methods of test- 

 ing milk should be published for the use of prac- 

 tical farmers. This need is well met by the present 

 edition of Mr. Walker-Tisdale's little book, which 

 has been enlarged and revised since the second 

 edition was noticed in Nature of August 10, 191 1. 



Letters to the Editor. 



[The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications .] 



The Energy of Cyclones. 



There can be no doubt, 1 suppose, that solar and 

 terrestrial radiation are ultimately responsible for the 

 kinetic energy of the winds. The doubts expressed 

 by Mr. R. M. Deeley in Nature of November 11 and 

 by Mr. W. H. Dines in the issue of November 18 can 

 refer only to the details of the phenomena consequent 

 on the process of transformation of the energy. The 

 first stage is obviously the storage of energy in the 

 potential form of air charged with heat and moisture 

 at the surface or lower levels and cooled by radiation 

 at high levels, especially in the polar regions, as on 

 the plateau of Greenland or on that of the .Antarctic 

 continent, or on the sunless slopes of the Himalaya. 

 Equally without doubt the next step is convection, 

 the greater part of which is indicated here and there 

 bv falling rain or snow. Measurements of rainfall 

 assure us that there is no lack of energy available for 

 violent winds if the heat-engine is at all efficient. 



The general effect of the process of convection is 

 the development of a vast circulation in the upper 

 regions of the atmosphere from west to east round 

 the poles, which has its counterpart in the normal 

 distribution of pressure at corresponding levels. That 

 is probably most pronounced at a level of 8 km., 

 because at that level density is equal all over the 

 globe at all seasons of the year, .\bove that level, up 

 to the level of equal pressure at 20 km. of which Mr. 



NO. 2666, VOL. 106] 



Dines writes, there is, on the average, a gradient of 

 density from the equator to the pole, and below the 

 level of 8 km. a gradient of density in the opposite 

 direction. The layer of maximum average velocity is 

 above the layer of maximum pressure-gradient on 

 account of the diminution of density with height. 



Below the level of 8 km. the distribution of pressure 

 is affected by the gradient of density in a very 

 irregular manner, because the distribution of land and 

 water is irregular. The net result at the surface is 

 the complicated distribution of average pressure 

 which we find in the maps of normals for sea-level. 



The maintenance of the average general circulation 

 from west to east in the higher levels is due to the 

 gradual convergence towards the polar areas from 

 which the cooled air flows. That must obviously be 

 balanced by a corresponding flow towards the equator, 

 and as poleward flowing entails a westerly circulation, 

 so flowing towards the equator entails an easterly 

 one. We must, therefore, find room in the system 

 for a body of air flowing from the east comparable 

 at least with the circulation from the west. We find 

 such a body of air in the great easterly circulation 

 of the intertropical regions, which is naturally stowed 

 away over the equator as far as possible from the 

 centres of the two polar demi-hemispheres of influence 

 of pressure-gradient. 



These great circulations, easterly and westerly, 

 form a normal " groundwork " of all atmospheric 

 motion ; and when Mr. Deelev and Mr. Dines write 

 of the energy of cyclones, they are not roncrned, I 

 think, with the enercrv of the general circulation of 

 the upper levels which I have described, but with the 

 minor circulations which represent the perturbations 

 of the major circulation. 



I think myself that the convect'on of warm, 

 moist air, combined with the vagaries of tem- 

 perature in the lower lavers, will, in the end, prove 

 to be sufficient to exniain the energy of cyclonic 

 air-currents — whether directly or as the secondary 

 effect of current-differences, t cannot say. Probably, 

 in order to get a correct view of the perturba'^ions. 

 we ought to subtract vertorially from the observed 

 winds the local motion of the normal circulation, or 

 else accustom ourselves more than we do to the 

 theoretical combination of local circulation with a 

 general circulation. 



There are four other aspects of th° nroblem uoon 

 which we are at present almost uninformed. The 

 first is the locality where the cyclone, which is the 

 subiect of study, was generated ; just as the cvclone 

 itself is a perturbation of the general circulation, so 

 what wo see goinjr on over our heads i« the per- 

 turbation of a cyclone which may have originated in 

 the £?eneral circulation thousands of miles away. A 

 cyclone is a more or less stable dynamical system 

 which certainly travels, but rhanpe'; as it travels. 

 The second aspect is the variation of velocity of the 

 wind with heipht in the general circulation and in the 

 cyclonic area itself. The third, which is closely con- 

 nected with the second, is th" traiectorv of convected 

 air. This could be calcvilated if we kn'^w the point 

 from which it started and the var-ation with height 

 of the current which carried it. One often r°pds of 

 convected air rising verticaUv. but we know that the 

 actual trai<-ctories of a nilot-balloon a>-e of very 

 various shapes, seldom vertical, and th» balloon may 

 part company from the air which suonorteH it at the 

 start by a distance measured in t'-ns of kilometres. 

 -Mr in convection rises very slowly. Tf we set its 

 vertical velocity at one-hundredth of that of a nilot- 

 balloon, the convected air may be thousands of kilo- 

 metres from the starting point before its upward 

 journey is finished, and its path may be very com- 



