Must he con- 

 fined to bllOlt 

 arts. 



ON DITIDING INSTRUMENTS. 



tiling; slippery iii the action, which would have been indi- 

 cated by raea^llnng tlie same part at ditiereut times differ- 

 ently, there would have been an end of it at oncej but* 

 this not being the case in any sensiV)le degree, the roller 

 becomes a useful auxiliary to fill up short intervals, the 

 limits of which have been corrected by more certain 

 means *. 



Bird, 



* There are two things in the foregoing account of the action of the 

 "roller, which have a tendency to excite surprise- The fir^t is, that the 

 roller sho'jld, in difiep.ut parts of its journey round the circle, measure 

 Porositv of the the latter so difFiM-'Utly. One would not wonder, however, if in taking 

 metal a source jhe measure across a ploughed field, it should be found different to a pa- 

 errour, rallel measure tken upon a gravel walk ; and, in my opinion, the cases 



are not very dissimilar. Porosity of the metal, in one part of fhe circle 

 more than in the other, must evidently have the same effect; brass uii- 

 hamnicred is always porous; and the part, which has felt the effect of 

 two blows, cannot be so dense as other parts which have felt the effect of 

 three; ai d, should the edge of the circle be indented hy jarring turnings 

 it would produce a visible similitude to ploughed ground. Every work- 

 man must be sufficiently upon his guard against such a palpable source 

 of errour ; yet perhaps with our grtatesl care we may not be able to avoid 

 it altogr ther. The second is, that notwithstanding the ineciuality above- 

 mentioned, the roller having reached the point upon the circle from which 

 it set out, should perform a second, ihird, &-c., course of revolutions, 

 without any sensible deviation from its former track ; this is not per- 

 haps so easily accounted for. It must be mentioned, that the exterior 

 border of the circle should be turned roundingy presenting to the roller a 

 convex edge, the radius of curvature of which is not greater than one 

 tenth of an inch. !Now, were the materials perfectly inelastic and im- 

 penetrable, the roller could only touch the circle in ajwint, and in pass- 

 The roller and '"g round the circle, it could only occupy a line of contact. This in 

 metal to be di- practice is not the case ; the circle always marks the roller with a broad 

 ^"^h ^h '° ''*'*' ^"^'^ thereby shows, that there is a yielding between them to a con- 

 siderable amount. The breadth of this list is not less than one fiftieth of 

 an inch; and it foUdws, that at least \2' of the circle's edge must be in 

 .contact at the same time-; that the two surfacss yield to each other in 

 ' depth, by a quantity equal to the ver. sin. of half that arc, or l-1800th 

 of an inch : and that the circle ;.as always hold of the roller by nearly 1* 

 of the edge of llie latter. Whoever has examined the surfaces cf metals, 

 which have rolled against each other, must have observed thut peculiar 

 kind of indentation, that always acc'om|)anies their action ; and there can 

 be no doubt, that the particles ©f a rolleT, and those of the surface on 

 v.hich it acts, which mutually indent each other, will, upon a second 



course 



