ON MACHINERY. ' 173 



yard long, united by joints to each other, and to two other pieces of a foot 

 each; one of the first pieces being fixed, if the shorter piece opposite to it 

 be produced to the length of four feet, its extremity will move at first in a 

 right line. The proportions of the rods may also be made more convenient 

 than these, and others may be added to them, if it be required, which may 

 make a line move so as to remain always in parallel directions, (Plate XIV. 

 Fig. 185 . . 188.) 



But of all the modes of communicating motion, the most extensively use- 

 ful is the employment of wheelwork, which is capable of varying its direc- 

 tion and its velocity without any limit. 



Wheels are sometimes turned by simple contact with each other; some- 

 times by the intervention of cords, straps, or chains, passing over them; and 

 in these cases the minute protuberances of the surfaces, or whatever else may 

 be the cause of friction, prevents their sliding on each other. Where a 

 broad strap runs on a wheel, it is usually confined to its situation, not by 

 causing the margin of the wheel to project, but, on the contrary, by mak- 

 ing the middle prominent: the reason of this may be understood by ex- 

 amining the manner in which a tiglit strap running on a cone would tend to 

 run towards its thickest part. Sometimes also pins are fixed in the wheels, and 

 admitted into perforations in the straps; a mode only practicable where 

 the motion is slow and steady. A smooth motion may also be obtained, with 

 considerable force, by forming the surfaces of the wheels into brushes of hair. 

 (Plate XV. rig. 189.) 



More commonly, however, the circumferences of the contiguous wheels 

 are formed into teeth, impelling each other, as with the extremities of so many 

 levers, either exactly or nearly in the common direction of the circum- 

 ferences; and sometimes»i.an endless screw is substituted for one of the 

 wheels. Informing the teeth of wheels, it is of consequence to determine 

 the curvature which will procure an equable communication of motion, with 

 the least possible friction. For the e(juable communication of motion, two 

 methods have been recommended ; one, that the lower part of the face of 

 each tooth should be a straight line in the direction of the radius, and the 

 upper a portion of an epicycloid, tliat is, of a curve described by a point of a 



