78 OF MILLS. 



the wheels and pinions of a clock, then the trundle 

 might divide the wheel exactly; that is to say, the 

 trundle might make a given number of revolutions 

 for one of the wheel, without a fraction. But as 

 any exact number is not necessary in mill-work, 

 and the cogs and rounds cannot be set in so truly 

 as to make all the intervals between them equal, a 

 skilful mill-wright will always give the wheel what 

 he calls a hunting cog ; that is, one more than what 

 will answer to an exact division of the wheel by 

 the trundle. And then, as every cog comes to the 

 trundle, it will take the next staffs or round, behind 

 the one which it took in the former revolution ; and 

 by that means, will wear all the parts of the cogs 

 and rounds which work upon one another equally, 

 and to equal distances from one another, in a little 

 time. 



The Method for setting out a Spur- Wheel and 

 Wallower. 



Draw the pitch lines A i, B 1, A % 2 B, (Plate 4. 

 fig. 1.) then divide them into the number of teeth 

 or cogs required, as a b c. 



Divide one of those distances, as b c, into seven 

 equal parts, as 1, 2, 3, 4, 5, 6, 7; allow three parts 

 for the thickness of the cogs, as 1, % 3, in the 

 cog a, and four for the thickness of the stave of the 

 wallower: one reason for allowing three parts for 

 the cog, and four for the stave is, the wallower is 

 in general of less diameter than the wheel, there- 

 fore subject to more wear, in proportion of the 

 number of cogs to the number of staves; but if 

 there is the same number of staves as of cogs, they 

 may be of equal thickness, as 1, 2, 3, 4, in the 

 stave m; (Fig. 2.) the height of the cog is equal to 

 four parts; then divide its height into five equal 



