Febeuaey 17, 1899.] 



SCIENCE. 



265 



and those iu the full glare of the sun. This 

 condition would direct the resulting electric 

 current from east to west, or iu the direction 

 opposite to that of the earth's rotation. '' 



The author says we have no good theory to 

 account for the earth's magnetism unless we are 

 ready to accept the one he has proposed. Let 

 us see, then, how the well known magnetic phe- 

 nomena of the earth are accounted for by this 

 theory. 



First. The north end of the compass needle 

 points approximately toward the north. Ap- 

 plying Ampere's rule to Trowbridge's currents 

 flowing in the upper regions of the air from 

 east to west we find that the north end of the 

 needle would point south. Hence the author's 

 currents must be reversed, i. e. , they must flow 

 from west to east, or in the same direction as 

 that of the earth's rotation. 



Second. The north end of the dip needle 

 points down in our latitude ; hence applying 

 Ampere's rule again, the electric currents must 

 go in the clockwise direction around the needle, 

 or, in other words, must proceed from east to 

 west, or contrary to the direction of the earth's 

 rotation. We should have, then, here a peculiar 

 state of things. In order to satisfy the phe- 

 nomena of the horizontal needle, Trowbridge's 

 currents must go from west to east ; to account, 

 however, for the known facts of the dipping 

 needle, they must simultaneously go in a con- 

 trary direction. 



In short, if electric currents produce the ob- 

 served phenomena of the compass and of the dip 

 needle they cannot be in the atmosphere, but must 

 be inside the earth^s crust and proceed from east to 

 west. Let the author apply Ampei-e's rule to 

 these currents and he will find that they will 

 now completely represent the known magnetic 

 phenomena. 



The fact that the causes of the earth's mag- 

 netism must be almost entirely within the earth's 

 crust was shown mathematicalty by Gauss half 

 a century ago and has been amply verified by 

 the recent investigations of Schmidt. His 

 elaborate mathematical analysis has resulted in 

 the following conclusions : 



The earth's magnetic force consists of three 

 parts, viz : (1.) The greatest part ; this is to be 

 referred to causes within the earth's crust, and 



possesses a potential. (2.) The smallest part 

 about 1-40 of the entire force ; this is due to 

 causes outside the earth's crust, and likewise 

 possesses a potential. (.3.) A somewhat larger 

 part than the preceding ; this does not possess a 

 potential, and, in consequence, points to the 

 existence of vertical earth-air electric currents. 

 These currents amount, on the average, for the 

 entire earth's surface, to one-sixth of an ampere 

 per sq. km. 



L. A. Bauer. 



University of Cincinnati. 



I AM much obliged to Professor Bauer for his 

 courteous criticism of my theory of terrestrial 

 magnetism, and I am inclined to give great con- 

 sideration to the opinion of such an authority 

 on the earth's magnetism. I imagined, how- 

 ever, that the electrical currents were largely 

 localized at the region of the astronomical poles 

 of the earth, and I supposed, also, that the 

 earth, as a whole, is para-magnetic. 



Fig. 1. 



According to my theory, poles M and N, 

 Fig. 1, might, perhaps, be formed in this mag- 

 netic matter, which would be competent to 

 produce both inclination and declination of a 

 magnet A B. Considerations of the earth's ro- 

 tation and the temperature of the air currents 

 led me to localize, so to speak, the electrical 

 action at the poles of the earth. It has always 

 seemed to me that Gauss' theory may be con- 

 sidered a mathematical theory, which would be 

 true, considering the limited number of obser- 

 vations he had to work with, whether we sup- 

 pose the earth's magnetic poles to be formed 

 by currents in the crust of the earth or by ro- 

 tary phenomena in the medium outside the 

 earth. 



John Trowbeidge. 



