478 TRAN3xiCTJ0NS OF THE AMERICAN INSTITUTE, 



\\\) — then he gets on and cracks up the horses again! Fact is, some- 

 thing's wrong with the boy, or the plaguey machine, I don't know 

 which. Why! I got on that thing with him and drove once roimd, 

 just to show him how to do it, and it went well enough — then I 

 crave him the lines, rode round once with him and it cut ri^ht 

 through without a bit of fuss. Then I got oft* and would you 

 believe me! that boy did not go twenty rods 'fore he was choked 

 up again as bad as ever!" 



" Well, Uncle Joe, I will go out to the field with you and take 

 a look at your machine — perhaps I can find out the difficulty. Two 

 heads, you know, ai'e better than one, if one is a — ." 



" Now see here! This machine has three varieties of motion — 

 progressive, rotary and vibratoiy. The machine, you see, follows 

 the horses across the field — that is progressive motion; the wheels 

 all turn round — that is rotary or circular motion; the knife moves 

 to and fro in the same channel — that is vibratory motion." 



" Yes, I see." 



"Now, which motion works the gearing and knife?" 



"The circular motion, I 'spose." 



" Very well; now suppose the circular motion ceases for only an 

 instant, what will happen?" 



" Why, the knife will stop, of course." 



" Well, what is it that keeps up the circular motion of these 

 driving wheels?" 



"Why, the horses, I 'spose." 



" Not exactly that, Uncle Joe. Suppose they were smooth-faced 

 wheels and on the ice, and you held them back a little with your 

 hand, would they roll on?" 



" No, they'd slide." 



" Then you see that it is friction, or, as it is generally called, 

 'traction,' on the ground, that makes them turn around." 



" Yes, I see." 



"It is the same thing that makes a locomotive wheel revolve and 

 enables it to draw the cars; but if you destroy that traction by 

 greasing the rail, or by removing the weight from the wheel, you 

 know what happens. It won't draw the load. Just here, too, is the 

 difficulty of making a raacliine that will move along with four or five 

 plows in the ground. Obed Hussey found this out at a great cost. It 

 c^n't be done. Now, 5'our machine weighs only five hundred pounds, 

 you say, and runs very light, only two hundred and fifty pounds draft. 

 Now, let us see what it is that makes this two hundred and fifty 



