EFFECTS OP PLY-WHEELS. 17i 



D, the rod, upon which one end of the lever in which 



the pulley H, is placed, turns. 

 M, the guides. 

 N, the arm, or lever, which guides the rest, against 



which the board to be cut, is placed. 

 Fig. 2. — The guides, separated from the machine. The guide 

 M, is raised up from the lever, showing a small pin on 

 the right end of it, which, when the guide is pressed 

 down, shuts into the hole below and moves the lever 

 by being pushed either way. 



For further particulars reference is made to John Howe and 

 Paul Pearson, the Inventors, Alna, Me, 



EFFECTS OF FLY-WHEELS. 



Sir, — Having, in the course of an extensive intercourse with 

 operative mechanics, frequent occasion to observe that very incor- 

 rect notions are entertained with respect to the operation of Fly- 

 wheels, I have thought that an explanation of the manrter of their 

 operation might profitably occupy a space in your pages ; and I 

 have, therefore, (in the hope that you will agree with me in think- 

 ing it worthy of insertion,) abstracted from an excellent article on 

 the subject, by Dr. Brewster, in his " Appendix to Fergusoii's 

 Lectures,'''^ as much as seems to me necessary to place the whole 

 subject in a simple and clear light before your readers. B. D. 



" A Fly, in mechanics, is a heavy wheel or cylinder, which moves 

 rapidly upon its axis, and is applied to machines for the purpose of 

 rendering uniform a desultory or reciprocating motion, arising 

 cither from the nature of the machinery, from an inequality in the 

 resistance to be overcome, or from an irregular application of the 

 impelling power. When the first mover is inanimate, as wind, 

 water, and steam, an inequality of force obviously arises from a 

 variation in the velocity of wind, from an increase of water occa- 

 sioned by sudden rains, or from an augmentation or diminution of 

 the steam in the boiler, produced by a variation in the heat of the 

 furnace ; and, accordingly, various methods have been adopted for 

 regulating the action of these variable powers. The same in-v 

 equality of force obtains when machines are moved by horses or 

 men. Every animal exerts its greatest strength when first set to 

 work. After pulling for some time, its strength will be impaired 5 

 and when the resistance is great, it will take frequent, though short 

 relaxation, and then commence its labor with renovated vigor. 

 These intervals of rest and vigorous exertion, must always produce 

 a variation in the velocity of the machine, which ought particular- 

 ly to be avoided, as being detrimental to the communicating parts 

 as well as the performance of the machine, and injurious to the ani- 

 mal which is employed to drive it. But if a fly, consisting either 



