364 Lord Rayleigh. Amount of .Oil necessary to [Mar.*27, 



45° to the axis, for in tliat direction the shearing stress is a maximum. 

 From this it would seem that the resistance overcome at rupture is 

 the resistance of the steel to shear. 



Experiments were made to see whether the resistance of steel to 

 direct shearing bore to its resistance to direct tension the ratio re- 

 quired by the above theory ; since the greatest shearing stress is equal 

 to one-half the longitudinal stress, we should expect to find the resist- 

 ance to direct shearing equal to one-half of the resistance to direct 

 tension. 



A series of experiments were made with the result that the ulti- 

 mate resistance to direct shearing was within, on the average. 3 per 

 cent, of the half of that to direct tension. 



The appearance of the fracture of steel bars is next discussed. It 

 would appear that when the stress is uniformly distributed in the 

 neighbourhood of the ruptured section, the fracture is at 45° to the 

 axis, the bar having sheared along that plane which is a plane of 

 least resistance to shear. The tendency to rupture along a plane of 

 shear may be masked by a non-uniform distribution of stress. 



Two plates of photographs are added, showing examples of steel 

 bars broken by shearing under longitudinal stress. 



IV. " Measurements of the Amount of Oil necessary iu order to 

 check the Motions of Camphor upon Water." By Lord 

 Rayleigh, Sec. R.S. Received March 10, 1890. 



The motion upon the surface of water of small camphor scrapings, 

 a phenomenon which had puzzled several generations of inquirers, 

 was satisfactorily explained by Yan der Mensbrugghe,* as due to the 

 diminished surface-tension of water impregnated with that body. In 

 order that the rotations may be lively, it is imperative, as was well 

 shown by Mr. Tomlinson, that the utmost cleanliness be observed. 

 It is a good plan to submit the internal surface of the vessel to a 

 preliminary treatment with strong sulphuric acid. A touch of the 

 finger is usually sufficient to arrest the movements by communicating 

 to the surface of the water a film of grease. When the surface-tension 

 is thus lowered, the differences due to varying degrees of dissolved 

 camphor are no longer sufficient to produce the effect. 



It is evident at once that the quantity of grease required is exces- 

 sively small, so small that under the ordinary conditions of experiment 

 it would seem likely to elude our methods of measurement. In view, 

 however, of the great interest which attaches to the determination of 

 molecular magnitudes, the matter seemed well worthy of investiga- 

 tion; and I have found that by sufficiently increasing the water 

 * 1 Memoires Couronnes ' (4to) of the Belgian Academy, vol. 34, 1869. 



