FRKT. 



FRICTION. 



lie 



4. C<raiy and tratufrmug 

 ouldy 



. . Frescoes are best cleaned 



with bread. The mouldy appearance which sometimes shows itself 

 ran be nunored with wet sponge, unless it arise from saltpetre in 

 the walU. in which caw there is no remedy. These injuries must be 

 prorided against in the conitruction of the walla : the vertical progress 

 of d unp may be intercepted by covering one of the lower courses of 

 bricka or stone* with aheet lead, which must be protected on each 

 nda by a coat of pitch ; the wall may be then continued u usual. 

 Freacoea may be safely washed with a soft sponge and water ; vinegar 

 and wine may likewise be used with safety ; even frescoes that have 

 been whitewashed over can be washed clean. The frescoes by Raffaelle, 

 in the Statue of the Vatican, though covered with nearly two centuries 

 of dirt, were walked with wine by Carlo Maratti. 



As many valuable frescoes, if left in their original localities, must 

 have inevitably peruked, through the bad construction of the walls, 

 unfit situation*, or from other causes, the invention of some mode of 

 emoving them from their objectionable situations became an object of 

 great interest, and several very ingenious methods of transferring 

 frescoes from walls have been devised, and often practised with 

 success. Mr. Ludwig Gruner removed and transferred to canvas is 

 1829, at Brescia, some frescoes by Lattanzio Gambara, in the convent 

 of St. Eufemia, in the following manner : the first process was to 

 clean the wall perfectly ; then to pass a strong glue over the surface, 

 and by this means to fasten a sheet of fine calico upon it. The calico. 

 being made fast by the glue to the irregularities of the wall, was itself 

 covered with glue, and some strong linen glued over it. " In this 

 state heat was applied, which caused the glue, even on the fresco, to 

 sweat through the cloths, and to incorporate the whole. After this a 

 third layer of strong cloth was applied on a new coat of glue. The 

 whole remained in this state two or three days (the time required may 

 vary according to the heat of the weather). The superfluous cloth 

 extending beyond the painting was now cut off so as to leave a sharp 

 edge : the operation of stripping or rolling off the cloths began at the 

 corners above and below, till at last the mere weight of the cloth and 

 what adhered to it assisted to detach the whole, and the wall behind 

 appeared white, while every particle of colour remained attached to 

 the cloth. To transfer the punting again to cloth, a stronger glue is 

 used, which resists moisture, it being necessary to detach the cloths 

 first used, by tepid water, after the back of the painting is fastened to 

 its new bed." 



Some frescoes by Paul Veronese, in the Morosini villa, near Castel 

 Franco, were transferred by Count Balbi of Venice. Cloth was 

 fastened to the wall by means of paste made of beer and flour, and 

 rivetted to the irregularities of the surface by means of a hammer 

 composed of bristles. 



Cicognara, Dtl Datacco delle Pitture a Fretco, 1825, in ' Antologia 

 di Firenze,' voL xviii. num. 52 ; Baruffaldi, Vita di Antonio Contri, 

 Pittore e Rilemtore di Pitture dal Muro, Venice, 1834 ; Pointer, Beitrdge 

 zur neuerii Kmu'getchickte, Leipzig, 1835; Cenni, Sopra diverse 

 Pitture tiaceate dot Muro e tnuportate u Tela, 4c., Bologna, 1840 ; 

 Eastkke, Material* tor a ffatory of Oil Painting, 1847, (chap, vi.); 

 C'untributintu to the Literature of the Fine Artt ; Mrs. Merrifield, Original 

 TreatiM* dating from the 12th to 18th centuries on the Arts of 7V 

 2 vols. 1849 ; Taylor, Manual of Pretco and Encauttic Painting, 1848 ; 

 the Appcndicet to the Report* of the Commiaioncrs on the Pine Artt, 

 from the 2nd Report published in 1842, to the llth published in 1858, 

 which contain much important matter connected with fresco-painting, 

 including many details on subjects to which there is not apace in this 

 article even to allude. 



FRET, in musical instruments of the stringed kind, is a wire fixed 

 in the neck, for the purpose of marking the exact part of the finger- 

 board to be pressed for the purpose of producing certain sounds. 

 Frets are now never applied to any instruments except gnitare, 

 lute., Ac. 



FRIARS, from the French freret, a term in strictness meaning the 

 brethren of a community, but more particularly applied to a new order 

 of religious persons, who mostly sprang up at the beginning i >f th<> 

 18th century, and were encouraged in the hope of restoring respect to 

 the monastic institution, the ample endowments of which had led it to 

 degenerate from its primitive austerity, and yield to luxury and 

 indulgence. 



The friars consisted of Dominicans, Franciscans, Carthusians, Cis- 

 tercians, Trinitarians or Maturines, Crossed or Crutched Friars, Austin 

 Friars, and Bonhommes or Good Men. These last were brought into 

 England by Edmund, earl of Cornwall, in 1288, and a colony of them 

 was placed at Ashridge in Buckinghamshire. The Capuchins and 

 Observants were distinctions of the Franciscan Friars. 



Accounts of the principal orders will be found under their respective 



FRICTION. The rubbing together of two substances, in the 

 course of which action (in mechanical operations) a resistance is 

 developed to the motion of those substance! The resistance of 

 friction is produced by the asperities, or unevenness, of the nililiing 

 surfaces; for however perfect the polish of those surfaces may be, 

 there are always irreguUrities of height, or of hardness, in the mole- 

 cules of the respective bodies of a nature to allow some of the particles 

 of the upper one to force themselves into the depressions, or softer 

 particles of the lower, from whence they cannot be removed without 



: - :.' d 



/' MM 



the exercise of a force opposed to the continuance of motion. At the 



:duy it U customary to consider Friction under two heads, the 

 of tHiliny, or that which is produced when the moving 

 constantly over the same part of the fixed surface, or ricr rend ; 

 and the Friction of rolling, or that which U produced when the parts 

 of the surfaces of contact are constantly changing their > 

 positions. 



Amontons, Bulfinger, Parent, Euler and Coulomb, were amongst 

 the earliest writers on physics who treated the subject of Friction, 

 and their researches have been confirmed and developed by the labour* 

 of Kergusson, Viuoe, Navier, Morin, Ac. The laws deduced by these 

 observers may be thus briefly stated. 



1. Friction is proportional to the normal pressure which the 

 surfaces of contact exercise on one another, but it varies according to 

 the nature and the state of the surfaces of contact, and is independent 

 of the velocity of movement, or of the extent of the surfaces. The 

 above law is, however, limited in this sense, namely, that when the 

 pressure exceeds a certain point, the surfaces are as it were driven into 

 one another, and the friction of the bodies in motion increases without 

 any apparent rule ; it becomes in fact abrasion. 



'.'. When bodies have been for some time in contact, especially if 

 they should be of a compressible nature, the (sliding friction will be 

 found to be greater at the commencement of motion than at a subse- 

 quent period. A slight blow in the direction of the intended motion 

 will produce a sufficient movement to obviate the effects of this 

 temporary increase of resistance. The coefficient of friction is the 

 term applied to the force which it is necessary to exert in order to 

 me the resistance directly opposed to motion ; and, as it U pro- 

 portionate to the normal pressure exercised by the two surfaces in 



contact, it is expressed by the simple formula /=-, in which /=the 



coefficient required, F = the resistance ascertained experimentally; and 

 p = the pressure. Elaborate tables of the coefficients of sliding and 

 rolling friction have been drawn up by Morin ; and others are quoted 

 by Poncelet in his ' Introduction a la Mecanique Industrielle,' see also 

 Claudel, ' Formules a Vusage des Ingenieurs/ Ac. 



3. The interposition of an unguent of any description modifies these 

 laws in a very remarkable manner, so that it is necessary to calculate 

 upon a particular coefficient of friction dependent upon the nature and 

 state of the unguent, and upon its mode of supply, whenever any of this 

 class of materials are used. 



4. The friction of cylinders rolling upon a horizontal plane is in the 

 direct ratio of their weights, and in the inverse ratio of their 

 diameters. 



5. In all cases in which solid bodies rest upon one another, there U 

 an inclination of the surfaces of contact at which the bodies are sus- 

 ceptible of motion by the mere effect of gravity ; and it is of the 

 utmost importance in all the operations of applied mechanics to ascer- 

 tain the precise value of this ai>gle of inclination, or, as it is sometimes 

 called, of this lim <>f refinance. Morin's tables contain a 

 series of practical observations on this subject ; they have been trans- 

 lated in Moeeley's admirable work, 'The Mechanical Princij<' 

 Engineering and Architecture.' The thrust of earthworks upon re- 

 taining walls is a special illustration of thU problem, which will be 

 discussed under RETAINING WALLS. 



6. In practical mechanics, also, the effects of friction are in some 

 cases modified by the resistance offered by the rigidity of the materials 

 in motion ; as, for instance, in the case of cords working over pulleys, 

 or of bands working upon driving wheels. The particular laws of this 

 class of actions will be discussed under RIGIDITY OP CORDS ; and for 

 the present it may suffice to say that the increase of resistance occa- 

 sioned by the cord or band may be represented by a formula of two 

 terms, one of which is a constant quantity, and the other the product 

 of a constant quantity, or factor of the resistance divided by the re- 

 sistance itself : so long, at least, as the same cord only is concerned. 

 For different cords, the constants vary, within certain limits, as the 

 squares of the diameters or of the circumferences of the cords, in 

 respect to new cords, wet or dry ; in respect to old cords, they vary 



nearly as the - power of the diameters, or of the circumferences. The 



rigidity of cords only acts to increase the resistance in the portions of 

 their length where they wind upon the pulleys. 



Friction is considered, and sometimes practically resorted to, as a 

 source of heat, for the inhabitants of the South Sea Islands obtain fire 

 by rubbing two pieces of dry wood violently against each other. The 

 heat developed by the friction of machinery in motion is one of the 

 most serious causes of its deterioration ; and it is n tln> account, 

 nearly as much as upon account of the motive power lost through the 

 resistance of friction when no lubricating materials are used, that it u 

 necessary to provide for the continuous supply of fresh and cool 

 lubricators. It may occasionally happen that the pressure, or weight, 

 of the moving body may force out the greases ordinarily used from 

 between the bearing surfaces, in which cases, heating, friction, or even 

 abrasion, may ensue. It is thi-ref-ir.- inijwrtant to proportion the area 

 of the bearing surfaces to the pressure in such wise as to prevent its 

 attaining this limit ; and, under any circumstances, to use the most 

 fluid lubricators possible, as they will most easily be forced into the 



