Notices respecting New Boohs. 309 



The critical loads observed were — 



5885, 6015, 5897, and 5800 grammes, 

 Mean 5899 grammes. 



The critical load calculated by (6), the length I being 110 cms., 

 is 5732 grammes. 



Tested as the beam considered in (7), with a single central 

 load, and the ends free to rotate in azimuth, the observed 

 critical loads, the beam being inverted for the second 

 experiment, were 



24,098 and 24,303 grammes, 

 Mean 24,200 grammes. 



The critical load calculated from (7), I being 55 cms., is 

 24,258 grammes. As a check on the methods and apparatus, 

 the loads producing Euler's instability of a long column were 

 observed for two positions of the specimen. The results were 



11,570 and 11,470 grammes. 



The theoretical critical load is 11,270 grammes. 



The chief source of error in the experiments was the want 

 of uniformity in the thickness of the specimen, this dimension 

 varying from "2550 cm. to '2628 cm. in different parts. No 

 method of completely eliminating this cause of error suggested 

 itself; and since the stiffness varies as the cube of the thickness, 

 the divergence of the observed from the calculated results is not 

 greater than might be expected to arise from this irregularity. 

 Melbourne, May 1899. 



XXXIII. Notices respecting New Boohs. 



Electromagnetic Theory. Vol. II. By Oliver Heaviside. 



(The Electrician Series.) 



THIS second volume of Mr. Heaviside's ' Electromagnetic Theory 

 contains four chapters, the first being a very short one on the 

 Age of the Earth, a subject suggested to the Author by Professor 

 Perry. Mr. Heaviside attacks the well-known differential 

 equations relating to the flow of heat in spheres and solid 

 blocks by his operational method, on which, indeed, the whole of 

 this volume is founded, and of which (apart from physical concep- 

 tions) it may be said to be a continuous illustration. No definite 

 result as to the age of the Earth can be deduced, owing, of course, 

 to the uncertainty of the various thermal constants (or, rather, 

 parameters) involved, and the widest (or wildest ?) estimates are still 

 possible. For example, M. Eesal gives in his Physique Mathe- 

 matique an investigation which leads to the result that from the Coal 



