66 MECHANICS. 



most without friction. Small balances have been so 

 skillfully constructed as to turn with one thousandth 

 part of a grain, 



2. Levers of the second kind are less numerous, but 

 not uncommon. A handspike used for rolling a log is 

 an example. A wheel-barrow is a lever of the second 

 kind, the fulcrum being the point where the wheel rests 

 on the ground, and the weight the centre of gravity of 

 the load. Hence, less exertion of strength is required 

 in the arm when the load is placed near the wheel, 

 except where the ground is soft or muddy, when it is 

 found advantageous to place the load so that the arm 

 shall sustain a considerable portion, to prevent the 

 wheel sinking into the soil. A two- wheeled cart is a 

 similar example ; and, for the same reason, when the 

 ground is soft, the load should be placed forward to- 

 ward the horse or oxen ; on the other hand, on a smooth 

 and hard, or on a plank road, the load should be more 

 nearly balanced. An observance of this rule would 

 often save a great deal of needless waste of strength, 

 A sack-barrow, used in barns and mills for convey- 

 Fig. 45. ing heavy bags of grain from one 



part of the floor to another, is a le- 

 ver nearly intermediate between 

 the first and second kind, the 

 weight usually resting very near- 

 ly over the fulcrum or wheels. 

 When the bag of grain is thrown 

 forward of the wheels, it becomes 

 a lever of the first kind ; when 

 back of the wheels, it is a lever 

 'sach-barrow. of thc sccond kind. As it is 



