FARM MACHINERY 289 



wagon as the oil, which is always the fluid used, is pumped 

 into the cylinder below the piston. The hydraulic jack does 

 not raise the load with an even motion, owing to the intermit- 

 tent action of the pump. 



The screw and nut device acting upon the principle of the 

 screw jack has one bad feature, and that is the lack of pro- 

 tection of the screw from rust and dirt, as it must be bright 

 and well lubricated at all times. The rack and pinion is a 

 device used on at least two makes. This is connected to the 

 front end of a platform at .each corner, and the wagon is 

 raised on the platform. This method obviates the difficulty 

 of dumping wagons of long and short wheel bases. All jacks 

 should have a quick return motion for returning the wagon 

 to place. 



Wood and steel are used in the construction of the dump- 

 ing jack. Owing to the fact that this implement is usually 

 exposed more or less to the weather, during its season of use, 

 at least, the steel construction is to be preferred. If the jack 

 is to be moved from place to place often as conditions may 

 require, it should be provided with a truck, which most manu- 

 facturers will furnish at a slight extra cost. 



The Hopper and Elevator. The hopper, or elevator 

 extension, is made so as to be raised to a vertical position or 

 swung to one side so that the load may be driven into the jack 

 or dump. In the first type, to assist in lifting the hopper, 

 springs or a windlass should be, and usually are, provided. 

 The carrier may be continuous through the hopper and the 

 elevator, or a separate carrier or web may be provided for 

 each. The first arrangement permits the hopper to be placed 

 nearer the ground, but has a tendency to overload the chains 

 of the carrier, which, in many cases, have too much to do for 

 their strength. The low hopper is a decided advantage in 



unloading a low-wheeled wagon. 



10 



