FRI 



Fill 



tide, increasing by heavy rains, and flow- 

 ing out into the sea, which it often dis- 

 colours to a considerable distance from 

 the shore, so that the line which divides 

 the two colours may be perceived dis- 

 tinctly for a great length along the coast. 



FRET, or FRETTE, in architecture, a 

 kind of knot or ornament, consisting of 

 two lists or small fillets, variously inter- 

 laced or interwoven, and running at pa- 

 rallel distances equal to their breadth. 



FRET, in heraldry, a bearing composed 

 of six bars, crossed, and variously inter- 

 laced. 



FRET, in music, signifies a kind of stop 

 on some instruments, particularly bass- 

 viols and lutes. Frets consist of strings 

 tied round the neck of the instrument at 

 certain distances, within which such and 

 such notes are to be found. 



FRET work, that adorned with frets. It 

 is sometimes used to fill up and enrich 

 flat empty spaces, but is mostly prac- 

 tised in roof's, which are fretted over 

 with plaster-work The Italians also use 

 fret-works in the mantling of chimneys, 

 with great figures ; a cheap piece ot mag- 

 nificence, and as durable almost within 

 doors, as harder matters in the weather. 



FRICTION, in mechanics, the rubbing 

 of the parts of engines and machines 

 against each other, by which means a 

 great part of their effect is destroyed. 



It is hadly possible to lay down gene- 

 ral rules concerning the quantity of fric- 

 tion, since it depends upon a multiplicity 

 of circumstances, as the structure, firm- 

 ness, elasticity, &c. of the bodies rubbing 

 against each other. Some authors make 

 friction upon an horizontal plane, equal 

 to one-third of the weight to be moved ; 

 whilst others have found it to be consi- 

 derably less. Two objects must, how- 

 ever, be observed, viz. the loss of power 

 which is occasioned by it, and the con- 

 trivances which have been made, and are 

 in use, for the purpose of diminishing its 

 effects. A body of a horizontal plane 

 should be capable of being moved by the 

 application of the least force ; but this 

 is not the case, and the principal causes, 

 which render a greater or less quantity 

 offeree necessary for it, are, 1, the rough- 

 ness of the contiguous surfaces ; 2, the 

 irregularity of the figure, which arises 

 either from the imperfect workmanship, 

 or from the pressure of one body from 

 the other ; 3, an adhesion, or attraction, 

 which is more or less powerful, accord- 

 ing to the nature of the bodies in ques- 

 tion ; and 4, the interposition of ex- 



traneous bodies, such as moisture, dust, 

 &c. 



Innumerable experiments have been 

 made for the purpose of determining the 

 quantity of obstruction, or of friction, 

 which is produced in particular circum- 

 stances. But the results of apparently 

 similar experiments, which have been 

 made by different experimenters, do not 

 agree ; nor is it likely they should, since 

 the least difference of smoothness or 

 polish, or of hardness, or, in short, of any 

 of the various concurring circumstances, 

 produces a different result. Hence no 

 certain and determinate rules can be laid 

 down with respect to the subject of fric- 

 tion. Mr. Vince, who has done much on 

 this subject, infers, 1, That friction is an 

 uniformly retarding force in hard bodies, 

 not subject to alteration by the velocity, 

 except when the body is covered with 

 cloth, woollen, &c. and in this case the 

 friction increases a little with the velocity. 

 2, Friction increases in a less ratio than 

 the quantity of matter or weight of the 

 body. This increase, however, is differ- 

 ent for the different bodies, more or less, 

 nor is it yet sufficiently known for any 

 one body, what proportion the increase 

 of friction bears to the increase of 

 weight. 3, The smallest surface has the 

 least friction, the weight being the same. 

 But the ratio of the friction to the surface 

 is not yet accurately known. Mr. Vince's 

 experiments consisted in determining 

 how far the sliding bodies would be drawn 

 in given times, by a weight hanging freely 

 over a pulley. This method would both 

 show him if the friction was a constant 

 retarding force, and the other conclusions 

 above stated. For as the spaces describ- 

 ed by any constant force, in given times, 

 are as the squares of the times, and as 

 the weight drawing the body is a con- 

 stant force, if the friction, which acts in 

 opposition to the weight, should also be 

 a constant force, then their difference, 

 or the force by which the body is urged, 

 will also be constant ; in which case the 

 spaces described ought to be as the 

 squares of the times, which happened ac- 

 cordingly in the experiments. The fric- 

 tion, ctfteris paribus, increases with the 

 weight of the superincumbent body, and 

 almost in the same proportion. The fric- 

 tion, or obstruction which arises from the 

 bending of ropes about machines, is in- 

 fluenced by a variety of circumstances, 

 such as their peculiar quality, the tempe- 

 rature of the atmosphere, and the diame- 

 ter, or curvature of the surface to which 

 they are to be adapted. But when other 



