492 



REPOKT — 1886. 



Experience shows that the harder the material the less is the loss due 

 to rolling friction, no greater normal pressure being required. Some ex- 

 periments recently made show that two chilled iron spherical surfaces 6" 

 diameter rolling on each other give at about 1,500 lbs. normal pressure 

 a coefficient of friction of over '25. For some time past expei'itnents have 

 been proceeding with the object of procuring perfectly sound chilled balls. 

 So far not one has been obtained, though some twenty to thirty have 

 been cast. 



3. Data derived from Experiments with Sphere Hoist and with 



Special Apparatus. 



The hoist shown in figs. 1 and 2 has been in daily use in the works 

 of Mr. Edward Shaw at Bristol for more than six months. Two sets of 

 experiments have recently been made upon it, the results of which are 

 given below (Tables I. and II.), and have also been plotted (Plate IX.). 

 In both cases the driving-belt was replaced by a cord on which 

 weights were hung, so that the force required to raise the load could be 

 ascertained. The first set of experiments (Table I.) shows the forces 

 required to overcome the friction of the machine, and raise the load from 

 a position of rest. The second set of experiments shows the forces re- 

 quired when the machine is in motion to lift the load at a constant speed. 

 In both cases the efficiency of the machine has been worked out for every 

 experiment in the following way : — 



Let W = load on hoist in lbs. 



= weight hanging over the driving-pulley. 



distance moved through by P 



^ velocity ratio = -j^— 3-: —. 



•' distance moved m same time 



P 

 V.R. 



by W 



Then efficiency=>?="g^y.R.=p ^^ 



TABLE I. — Statical Resistance of Friction. Experiments on Sphere Hoist 



AT Canons Marsh, Bristol, April 13, 1886. 



Weight of pan, 22 lbs. I Diameter of small pulley, 8| in. 



Diameter of large pulley, 17 in. | „ driving „ 13| in. 



