30 



MECHANICS. 



" The preceding Table contains the 

 results of experiments on the fqction of 

 axes of iron in boxes of copper. The 

 axis used was 1 9 lines in diameter, and 

 had a play of 1.75 lines in the copper 

 box; the pulley was 144 lines in dia- 

 meter, and weighed 14 Ibs. 



"The chief object in these experiments 

 was to determine the friction of axis 

 in motion. Coulomb, therefore, caused 

 the weights to run over a space of six 

 feet, and measured separately by half 

 seconds, the time employed to run over 

 the first and last three feet. 



" The weights employed to bend the 

 cord, and which are contained in the 

 fourth column, were calculated from the 

 tensions expressed in the fifth column, 

 by means of the formulae already given, 

 and the results of some previous expe- 

 riments. These weights being sub- 

 tracted from those of the sixth column, 

 which put the system in motion, leave 

 the weights employed in overcoming 

 the friction. These latter weights act- 

 ing at a distance from the centre of 

 rotation equal to the sum of the radii of 

 the pulley and the cord : the friction 

 which is exerted upon the axis, and 

 which, in the case of a very slow motion, 

 may be considered as making an equi- 

 librium with those weights, is therefore 

 equal to the product of those weights 

 into the ratio of the sum of the radii of 



the pulley and the cord, to the radius of 

 the axis, which ratio is very nearly 7 to 

 1, w r hen the w r eight is suspended by a 

 thin packthread, and nearly 7-2 to 1, 

 when it is suspended by the cord No. 1. 

 From these considerations the ninth 

 column was calculated. The weights 

 comprised in the eighth column are 

 composed, 1 . Of the weight of the pul- 

 ley or cylinder ; 2. Double the corre- 

 sponding weight in the fifth column ; 

 3. The weights contained in the sixth 

 column ; for the sum of these evidently 

 compose the pressure upon the axis. 

 Hence, to find the ratio of the friction 

 to the pressure, as expressed in the 

 tenth column, it is only to divide any 

 number in the ninth column by the 

 corresponding one in the eighth." 



" M. Coulumb has likewise endea- 

 voured to ascertain the friction of axes 

 of rotation made of the different kinds 

 of wood which are commonly found in 

 rotatory machines. To render the fric- 

 tion more sensible, he used pulleys of 

 12 inches mounted upon axes of 3 

 inches : sometimes the axes were im- 

 moveable ; at others, they moved, but 

 in both cases the friction was the same : 

 the proper precautions were adopted to 

 smoothen the surfaces in contact, and 

 thence to avoid the uncertainty and ir- 

 regularity which might otherwise have 

 attended the results. 



TABLE V. " Kinds of Wood used in the Experiments. 



Ratio of friction 

 to pressur*. 

 0.038 

 0.06 

 0.06 

 0.08 



Axis of holm-oak, box of lignum vitse, coated with tallow 



Ditto the coating wiped, the surface remaining oily 



Axis and box as before, but used several times without having the") 



coating refreshed J 



Axis of holm-oak, box of elm, coated with tallow .... 0.03 

 Ditto, both axis and box wiped, surfaces remaining oily . . 0.05 



Axis of box-tree, box of lignum vitse, coated with tallow . . . 043 



Ditto, the coating wiped, the surfaces remaining oily . . . 0.07 



Axis of box-tree, box of elm 0.035 



Ditto, the coating wiped off 0.05 



Axis of iron, box of lignum vita?, the coating wiped off, and the") 



pulley turned for some time J 



0.05 



" The velocity does not appear to 

 influence the friction in any sensible 

 manner, except in the first instants of 

 motion : and in every case the friction 

 is least, not when the surfaces are 

 plastered over, but when they are 

 merely oily. 



" The experiments on the stiffness of 

 cords already described, were made in 

 cases of motions nearly insensible j but 



M. Coulomb inquired whether with a 

 finite velocity the resulting effect of the 

 stiffness of the cord were augmented or 

 diminished. For this purpose he took 

 a pulley and box of copper, and an axis 

 of iron done over with tallow : the dia- 

 meter of the pulley was 144 lines, and 

 that of the axis 204 lines, and the cord 

 was one of 30 threads to a yarn, or No. 

 3, of which the stiffness with respect to 



