269 



mum being, for butt logs, at least, about one-third of the radius from the 

 center of the tree. 



A few weeks ago while making some tests of the strength of burr oak 

 and white oak, I was rather surprised to find a variation of tensile strength 

 of much greater amount than that obtained by Prof. Johnston for pine. 

 In the case of the white oak the strength varied from 12,000 pounds per 

 square inch at about one and a half inches from the surface of the tree to 

 about 24,000 pounds per square inch at a similar distance from the center. 

 The log was about ten inches radius and the variation was nearly uni- 

 form from the outside to the center. The burr oak showed a similar vari- 

 ation, but unfortunately the record of some of the tests, taken when I 

 was unable to attend personally to the matter, have been lost. I have 

 since made similar tests on water oak and on red oak. The results in the 

 water oak show no decided variation across the sections. The average 

 strength was about 14,000 pounds per square inch, and as nearly uniform 

 as is to be expected in tests of timber. The red oak was also much more 

 nearly uniform in strength across the sections than the white oak, but in 

 this case there was good evidence that the outside wood was the stronger, 

 especially on the side of the tree which had the larger growth. The vari- 

 ation in this case was from about 1-5,000 pounds on the square incii at the 

 center to 18,000 pounds on the square inch at the outside. The stronger 

 timber was, however, in this case, confined to about three inches of the 

 outer end of the radius. 



On AN AUTOGRAPHIC METHOD OF TESTIN(t THE MAGNETIC QUALITIES OF IRON. 



By Prof. T. Gray. 



[ABSTRACT.] 



At last Christmas meeting of the Indiana Academy I gave a brief de- 

 scription of some experiments which I had made on the magnetic quali- 

 ties of iron and of the results I had obtained in these experiments, which 

 were of a preliminary character. The general principle of the method was 

 to deduce the magnetic properties of the iron from the electro magnetic 

 inertia of a circuit composed mainly of a magnetizing coil surrounding 

 the iron. This electro-magnetic inertia is evidenced by the relative values 

 at each instantof the impressed e. m. f. on, and the current flowing through 



