PHYSICS. 93 



rnonstrated. It does not appear, however, to equal for ele- 

 gance of construction or for universality of application the 

 similar set of apparatus devised by the late Professor Wil- 

 lis, of Cambridge, and known by his name. This latter ap- 

 paratus, by the way, deserves to be much more generally 

 known in this country. As constructed by the Worcester 

 Free Institute, it is invaluable in demonstrating mechanical 

 principles. 



Kimball has published an important paper on the varia- 

 tion of friction with velocity, in which, curiously enough, he 

 harmonizes the statements of Morin and Coulomb, that the 

 coefficient of friction does not vary with velocity, with that 

 of Bochet, that it decreases as the velocity increases, and of 

 Hirn, that it increases as the velocity increases, simply by 

 showing that each is true at some given velocity. For very 

 low velocities the coefficient is small ; it increases at first 

 rapidly, then slowly, until at a certain rate of speed it 

 reaches a maximum ; beyond this point increase of velocity 

 decreases friction. The results of the above-named experi- 

 menters are explained by showing that Morin and. Coulomb 

 operated at velocities where the coefficient is near a maxi- 

 mum, and so obtained constant results ; while Bochet oper- 

 ated at high and Hirn at low velocities. Kimball used in 

 his measurements sliding friction down an inclined plane, 

 sliding friction at uniform velocities on a horizontal plane, 

 friction of belts on the surface of cast-iron pulleys, and fric- 

 tion of wrought -iron journals in boxes or bearings of dif- 

 ferent materials. The practical bearings of his results are 

 highly important. 



Sir William Thomson has described in Nature the results 

 which he has obtained with his new astronomical clock, de- 

 vised in 1869 with a view to improve both the compensa- 

 tion for changes of temperature and the form of escapement. 

 The latter is a modified Graham's dead-beat escapement, the 

 escapement-wheel consisting of only one tooth, being simply 

 a piece of fine steel wire attached to a collar fitting loosely 

 upon the shaft, and driven by friction from it, the shaft be- 

 ing connected with a suitable train of wheel -work with 

 uniform motion, moving a trifle faster than the keeping 

 of accurate time requires. To the lower portion of the 

 pendulum -bob two pallets are attached, near the end of 



