730 Notices respecting New Books. 



equilibrium the Thermodynamic Potential is a minimum, and 

 though Available Energy may be defined for all states, in 

 order to apply the test of its being a minimum it is neces- 

 sary to express it in terms of other functions, while such 

 expressions are valid only for states which may be obtained 

 in a reversible manner. 



LXX. Notices respecting New Books. 



A Magnetic Survey of Japan reduced to the Epoch 1895-0 and the 



Sea Level, carried out by order of the Earthquake Investigation 



Committee. Reported by A. Taisakadate, Professor of Physics, 



Imperial University, Tokyo. (Forming Vol. XIV. of the Journal 



of the College of Science . . . Tokyo.) Text pp. xii-}-180, with 



1 plate ; Appendix pp. (iv) + (347), with 87 plates and 11 maps. 



Tokyo, 1904. 



TX carrying out this important survey Prof. Tanakadate had the 



assistance of fifteen Japanese men of science who devoted the 



summers of four years, 1893 to 1896, to the work. The instruments 



employed — shown in a plate at the beginning — preseut several 



novelties, notably the silk suspension of the collimator magnet 



described on p. 13. 



In the absence presumably of data from fixed observatories, the 

 diurnal variations were eliminated by multiplying observations 

 throughout the day. The declination observations were so nume- 

 rous at each station as to give curves of the diurnal variation 

 occupying 59 plates in the Appendix. The other plates show the 

 exact position and surroundings of each of the 320 stations. The 

 method of reducing the observations to a common epoch is 

 unusual. The declination, for instance, at a place whose latitude 

 is ^ and longitude X is assumed to be given by a formula such as 



ci = a o + A(^-0 o )-hB(\-X o ) + C(X-X o )^ 



where the suffix refers to a certain central latitude and longitude. 

 The constants A &c. being determined from the present survey 

 and a previous one by Profs. Knott and Xagaoka, and so for two 

 different epochs, results are obtained for the secular change as 

 depending on (f> and X. The values thus found for the mean 

 annual change at different parts of Japan vary from + 3'*82 to 

 — 1'*85 in Declination, from -r-2''10 to — 4'* 17 in Inclination, and 

 from +29*97 to — 7*9y in Horizontal Force. Even allowing for 

 the fact that a range of 16° of latitude and 18° of longitude is 

 covered, the variety in these results is remarkable, and confirmation 

 from fixed observatories seems highly desirable. 



The question of the existence of a vertical earth-air electric 

 current is minutely considered. Table xviii. gives the results ob- 

 tained over Japan from formulae of the type 4nw = dY/dsc—dX./dy, 

 and analogous results for Great Britain and Austria are deduced 

 from the surveys of Eiicker & Thorpe and of Liznar. In all three 



