FORM OF TEETH OR COGS. 81 



contact with each other, they act 

 and thus a part of their force is lost, and they continue 

 scraping together with a large amount of friction so 

 long as they remain in contact. These eifects are 

 avoided hy giving to them the curved form represented 

 by b. Here, instead of pressing each other obliquely, 

 they act at right angles, that is, not obhquely ; and in- 

 stead of scraping, they roll over each other with ease. 

 These curves are ascertained by mathematical calcu- 

 lation, which can not be here given ; it may be enough 

 to state that they should be so formed that the points 

 in contact shall always work at right angles to each 

 other. For ordinary practical purposes, however, they 

 Fig. 61. may be made as is 



_ ^.- '/''■■--, i y''^\ "X— , shown in the annex- 



/ "">( /K V" \ ed figure {Fig. 61), 



! the teeth thus form- 



Mode of giving the bestform to cogs. ed arc rcmoved, leav- 



ing a blunt extremity, according to the figure. 



There are a few other rules that should always be 

 observed in constructing wheel- work, in order that the 

 wheels may run easily together, without jerking or 

 ratthng, the most important of which are the follow- 

 ing: 



1. The teeth must be of uniform size and distance 

 from each other, through the whole circumference of 

 the wheel. 



2. Any tooth must begin to act at the same instant 



D2 



