224 



SCIENCE 



[N. S. Vol. XXXVII. No. 945 



humidity and the actual temperature. In 

 some cases the authors prefer to use, instead 

 of the sheets, the heights of the surfaces of 

 equal pressure drawn at intervals of one tenth 

 of an atmosphere. Less accurate results can 

 also be found from observations at the earth's 

 surface, assuming average values for the 

 changes as one ascends vertically. By these 

 two methods it is possible to draw level sur- 

 faces and vertical sections that show the pres- 

 sure and density at diilerent parts of the 

 atmosphere. If the equilibrium were exact 

 the surfaces of equal pressure, of equal density 

 and the level surfaces would coincide. The 

 more these surfaces differ from each other the 

 greater the tendency to motion. Similar con- 

 siderations are discussed for the oceans, but 

 these do not at present appear to have the 

 same importance as the atmosphere. The 

 second part takes up the representation of 

 velocity. From observations of small balloons 

 the horizontal velocity in different localities 

 and at diilerent heights can be found. These 

 results are averaged with respect to height for 

 the sheets of the atmosphere that were used 

 in the first part of the work. The lines of 

 flow can now be drawn for each sheet and also 

 the curves of equal velocity. In some cases 

 the authors use " isogonal " curves, namely, 

 curves of equal direction of velocity. The 

 use of these curves to solve differential equa- 

 tions is credited to Sandstrom. It appears, 

 however,^ to be due to Massau, who called 

 them " isoclines." Towards the end of the 

 volume these diagrams are used to deduce the 

 vertical motion of the atmosphere under the 

 assumption that momentum is a solenoidal 

 vector, a downward velocity indicating pre- 

 cipitation. A supplement of 60 excellent 

 maps comprises conical projections of the 

 earth's surface showing the contour lines, in 

 24 sheets, and examples of the preceding 

 methods applied to actual cases. The middle 

 of Part II. contains an elegant study of two 

 dimensional vector fields and of the graphical 

 treatment of the operations which occurs in 

 the differential equations of hydrodynamics. 

 * See D 'Ocagne, ' ' Calcul Graphique. ' ' 



It is to be hoped that the authors will com- 

 plete their work by a third part, on the 

 dynamics of the atmosphere, as distinct from 

 the kinematics, including Professor Bjerk- 

 nes's own work on this part of the subject, 

 and also extend the period of six hours, which 

 is the limit of their prognostications, at pres- 

 ent. A fuller treatment of the thermody- 

 namics of the atmosphere would also be 

 desirable. 



P. E. Sharpe 



The Theory of Experimental Electricity. By 

 W. C. D. Whetham, F.E.S. Second edi- 

 tion. Cambridge, The University Press; 

 jSIew York, G. P. Putnam's Sons. Pp. xi -(- 

 340. $2.50 net. 



It gives us great pleasure to welcome a new 

 edition of Mr. Whetham's text-book. This 

 work presents the subject of electricity as a 

 living science and is characterized by a won- 

 derful freshness of treatment. It is thor- 

 oughly up to date and includes such matters 

 as the thermodynamic theory of electrolytic 

 cells, conduction through gases, radioactivity 

 and the electron theory. Although of neces- 

 sity brief, the treatment of these subjects is 

 quite accurate. Excellent judgment has been 

 shown in the choice of material and the newer 

 branches of the science are exhibited in their 

 proper relationship to the old. One of the 

 striking features of the book is the combina- 

 tion of simplicity and accuracy in the proofs 

 used in establishing the important principles 

 of the science. 



We regret to observe that two blemishes 

 which we noticed in the first edition have not 

 been removed. The treatment on pp. 35 et seq. 

 of the force due to a charged plane as though 

 the charge resided on both sides of the plane 

 is cumbersome if not actually misleading. 

 The proof on page 105, of the mechanical 

 force on a current due to a magnetic field, is 

 fallacious as it stands; although it is made to 

 give the right result. This matter could 

 easily be rectified without changing the 

 method. We also think that it is high time 

 that Franklin's proof that the charge resides 

 in the dielectric were dropped from the text- 



