CATALOGUE. PHILOSOPHY AND ARTS, PRACTICAL MECHANICS. 171 



sides a small force, of a pound for evety 30 square inches, 

 independent of the weight. After some time the friction be- 

 comes — or — • When olive oil is used, the friction bc- 

 10 9 



comes at once _L, with old soft grease about — . 

 6 7 



In order to examine the friction of bodies in motion, the 

 adhesion was destroyed by shaking the apparatus a- little. 

 When oak moved on oak in the direction of its fibres, the 

 friction was nearly the same in all degrees of velocity ; but 

 when the surfaces were large, the friction increased a little 

 with the velocity, and was diminished as the velocity in- 

 creased when they were small. For a pressure from 100 to 



4060 pounds on a square foot, the friction is about — , be- 

 sides a resistance of about 14 pound for each square foot, 

 independent of the pressure, increasing a little with the 

 velocity, occasioned jjerhaps by a down on the surfaces. 

 When the surface is very small, the friction is somewhat 

 diminished. The narrow surface being cross grained, the 



friction was invariably—. For oak on fir, the friction was 

 10 



— : for fir on fir — for elm on elm — , but varying ac- 

 6.3 6 10 



cording to the extent of surfiice; for iron or copper on 



^ood — , which was at first doubled by increasing the velo- 

 13 



city to a foot in a second, but after continuing the operation 



for some hours, was again diminished. For iron on iron 



, on copper, , after long attrition, — , in all velo- 



3.55 4.15 6 



cities. 



When an unctuous substance was interposed between 



surfaces in motion, the hardest was found to diminish the 



friction most, where the weight was great. Tallow being 



applied fresh from time to time to oak, the friction was 



' 28 



of the pressure, besides an adhesion of 1 pound for every 38 



square inches, when the velocity was insensible, or for every 



20 or 24 inches, when the velocity was afoot in a second. 



When the surfaces are very small, the tallow loses its effect, 



and the friction becomes — or — ; it is also increased by an 

 17 10 



increase of velocity, and by the substitution of soft grease for 



<allow. When the surfaces were soaked in grease, and 



wiped, the friction was about — , the adhesion 7 pounds for 



14 



a square foot. The narrow surface being placed across the 

 fibres of the fixed board, and drawn in their direction, the 

 effects were nearly the same, but more regular. The inter- 

 position of tallow has the greatest effect where wood and 



metal move slowly on each other ; thus the friction of iron 



I 1 



on oak becomes —, of brass on oak— ; but after 15 expc- 



liments, more than three times as much force v\-as necessary 

 to continue the motion ; so that tallow not frequently re- 

 newed appears to be injurious : when the surfaces are small, 

 it has little effect. For narrow surfaces of wood, moved 



1 

 on iron across the grain, the friction was — in all velocities. 



14 ^ 



In cases also where the operation has been long continued, 



as in all machines, the friction is independent of the velo- 



cit}'. For iron on iron, with tallow, the friction was — , the 



10 



adhesion 1 pound for 15 square inches ; on copper — , the 



adhesion l pound for 13 square inches; with oil or soft 



1 

 grease, the friction of iron or copper was , without any ad- 

 dition for adhesion. Whenthesurfacc of iron, moving on cop- 



1 1 



per, was small, the friction was -— with tallow, — With oil, 



in all velocities. On the whole it appears, that in the case of 



1 

 most machines, — of the pressure is a fair estimate of the 



friction. 



The next subject investigated is the rigidity of ropes. 

 This was supposed by Amontons and Desaguliers to vary as 

 the diameter, as the curvature, and as the tension. Cou- 

 lomb finds the power of the diameter expressing the rigidity 

 to be generally 1.7 or 1.8, never less than 1.4, and that a 

 constant quantity must be supposed to be added to the 

 weight. Wet ropes, if small, are a little more flexible 

 than dry ; if large, a little less flexible. Tarred ropes are 

 stiff'er by abouti, and in cold weather somewhat more. The 

 stiffness of ropes increases after a little rest. 



A rope of three strands, each of 2 yarns, 12^ lines in cir- 

 cumference, 8 inches of which weighed 2^ gros, and conse- 

 quently 125 grains E., being bent on a fixed axis 4 inches in 

 diameter, required a constant force of j^ pound Fr., and 



.; of the weight, to overcome its rigidity. The same 



rope tarred required J paund, and —of the weight. " The 



strands being of 5 yarns, the circumference 20 lines, and 

 the weight OJ gros, the rigidity was equal to 1 pound, and 



of the weight ; when tarred, the rope required 1 pound 



and of the weight to move it. With strinds of 10 



21 >4 



yarns, a circumference of 28 lines, and a weight of 12^ gros 



for 6 inches, the untarrcd rope showed a rigidity of 2 



2 



