78 



MECHANICS. 



good judgment before ho had incurred all the expense and 

 losses of unsuccessful trials. 



Even so simple a form as that of an ox-yoke is often 

 made unnecessarily heavy. Fig. 91 represents one that is 

 faulty in this respect, by having been cut from a piece of 



timber as wide as the dotted lines a c / and being thus 

 weakened, it requires to be correspondingly large. Fig. 

 92 is equally strong, much lighter, and is easily made from 

 a stick of timber only as wide as a h in the former figure. 

 In the heavier machines, it is necessary to knoAV the de- 

 gree of taper in the difterent pnrts with accuracy. A 

 thorough knowledge of science is needed to calculate this 



with precision, but a supei-ficial idea may be given by cuts. 

 If a bar of wood, formed as in a (fig. 93), be fixed in a 

 wall of masonry, it will possess as nmch strength to sup- 

 poit a weight hung on tlie end as if it were the same size 

 throughout, as h. The' first is equally strong witli the 

 second, and much lighter.''- The same form doubled must 



* The simple style of tills M'ork precludes on cx]ilan:\tlon of Ihc mode 

 of caleulatiou for determinini;: the exact fonn. Where the stick tapers 

 only on one side, it is a common parabola; if on all sides, a cubic 

 parabola. 



