230 



OF THE PARTS OF 



[SECT. vn. 



TABLE OF THE VARIATION OF ROTARY FORCE, WHEN A CRANK is IMPELLED 



BY A CONSTANT FORCE. 



The length of the crank is supposed to be 1 ; and the table applies to any other 

 length of crank, when the connecting rod is 2, 3, 4, 5, 6, or 7 times its length : 

 the columns below these numbers show the rotary force corresponding to the posi- 

 tions indicated in the first and second column. The stress in the last column being 

 greatest at each end, will have a tendency to wear the shaft into the form of an 

 oval, having its longest diameter at right angles with the connecting rod. 



PARALLEL MOTION. 



488. The next method to be described for communicating motion from a 

 piston rod to a beam is that called the ' Parallel Motion.' It was invented by 

 Mr. Watt, who first gave a notice on the subject in ' Robison's Mechanical Philo- 

 sophy ;' and its theory has been since analytically investigated by Prony. We shall, 

 by confining the enquiry to practical conditions, however, be able to treat it more 

 briefly, and show how the best proportions for practice may be obtained. 1 



There are two cases, which for simplicity we shall investigate separately, 

 though they are generally both in use in the same engine. 



489. FIRST CASE. If each of two bars A B, C D, Plates x. (A) and (B) Fig. 4. has 

 an axis at one end, round which it moves, and the other end be connected with a third 

 bar B D, by moveable joints ; then there is a point E in the middle bar which will 

 nearly describe a straight line. The rectilinear movement of the air-pump rod, in 



1 A patent for the protection of the Parallel Motion was taken out by Mr. Watt in 1784, and 

 the invention was in 1787 applied in its most perfect form to the engines of the Albion Mills. 



