430 On the Changes of Colour 



the inside of the hole through it being, as shown in fig. 4, 

 turned out concave, in tlie latter rather larger than the 

 edges, somewhat like the rim of a spectacle irame ; then 

 the internal circle M, being accurately fitted into it, is 

 swelled or bulged out by means of a taper mandrell, driven 

 into the hole in its centre to such a size that it will fill ihe 

 outer ring or circle exactly, in the manner of fig. 4, and 

 then it cannot get out of its place sideways, because the 

 interior circle exactly fills the groove or hollow parts formed 

 round wiihin side the annular saw K : this internal circle 

 M thus becomes the axis on which ihe saw turns. The 

 circumference of the saw, as shown in fig. 5, is notched. all 

 round with fine teeth, which perform the cutting, and al 

 intervals it is cut with deep notches, into which the pins 

 on the face of the wheel c are received, and act upon the 

 rino- so as to turn it round : the interior ring M, or axis of 

 the saw, is supported by being screwed to a piece of iron, 

 N, which also carries the centre pin of the wheel d, and is 

 itself screwed to the inside of the brass plates FG of the 

 frame, by the screws shoAn in fig. 1. W is a handle to 

 guide and direct the saw, moving it upon its several joints 

 B and E into any required position ; o is a spring which 

 in certain positions balances the weight of the frame FG, 

 &c. depending upon the joint B : P, fig. 2, is a gauge con- 

 sisting of a flat slip of iron, PS, which is fitted to the 

 imderside of the frame; it has a groove formed in it through 

 which a screw passes, ar:d the nut Q will fasten it at any 

 required point ; it is moved sideways by the screw R, and 

 adjusted to advance to any required distance towards the 

 extreme end of the saw : its use is to regulate the depth to 

 vhich the saw shall penetrate in cutting. 



The verv singular properly of this annular or circular saw, 

 in cutting deeper than its centre, renders it liiccly to prove 

 of great utility in a variety of surgical and mechanical ope- 

 rations. 



LXV. On the Changes of Colour produced ly Heat in. 

 coloured Bodies. By M. Gay-Lussac*. 



J. HE various colours presented by the different bodies of 

 nature present variations in their shades, and frequently 

 pass tiom one tint to another when they are exposed to a 

 certain temperature. There would be nothing remarkable 

 in these changes, if they were owing to a chemical altera- 



• Aimales de Chimie, August 1812. No. S'lS, p. 171. 



tion J 



