96 



MECHANICS. 



[ Lessen XTL 



called the governor wheel, which regulates 

 the velocity of the machine with which it is 

 connected. Thus, in the steam-engine it 

 acts upon the valve ; in the water-mill 

 upon the shuttle ; and in the windmill, 

 upon the sail-cloth. In the steam-engine 

 it consists of two heavy balls, A A, attached 

 to the ends of two rods, 

 which play upon a point at 



B, and as they rotate upon 



C, and separate by the 

 centrifugal force, a ring 

 above B is depressed, 

 which acts by means of a 

 rod upon the valve and 

 closes it, thereby dimin- 

 ishing the speed ; and as 

 the balls fall or are at rest, 

 the valve is opened, and 



the speed again established by supplying 

 more steam. 



154. T. What is a crank ? 



P. A mechanical contrivance, by means 

 of which a revolving motion is changed 

 into an alternate motion. 



155. T. How is this effected? 



P. [Experiment 9.] I have a piece of 

 iron wire, which you observe is straight. 

 I will now bend it so as to form a crank 

 thus, and you will easily 

 understand that while the 

 " B axis (A) revolves, the part 

 B, which is out of the same 

 line, will describe a circle (makes it do so 

 by twisting the wire between the finger and 

 thumb). Now if a piston or rod is attached 

 to the part B, its motion cannot be the 

 same as the axis, but must rise and fall, or 

 be alternate. 



Fiy. 49. 



156. T. I thought that cranks were 

 usrd to make alternate motions change 

 into revolving motions. Explain this 

 difficulty. 



P. Sometimes they are employed for 

 that purpose, and are double, being made 



Fig. 50. 



to turn two wheels as in Fig. 50, but the 

 same arrangement may be retained, and 

 only one crank used, which is the same as 

 turning the handle of an organ, the piston 

 or rod being represented by the hand. 

 Now as the piston's motion is alternate, or 

 falls and rises, the crank converts that 

 motion into a rotary one, and conveys it 

 to the wheels. We observe this in turning- 

 lathes and steamers. 



GENERAL QUESTIONS ON LESSON XII. 



1. How is motion transmitted to ma- 

 chinery ? 



2. Can motion be changed in its direc- 

 tion ? 



3. What is the difference between a 

 crown wheel and a bevelled wheel ? 



4. How may friction be partially pre- 

 vented ? 



5. Is force susceptible of accumulation ? 



6. What is the governor wheel ? 



7. What is the use of a crank ? 



8. Are cranks single or double ? 



