MECHANICS. 



53 



in the machine, the baUs C C' recede 

 from the axis, draw down the joint E, 

 partially close the valve, and check the 

 supply of steam to the cylinder. The 

 impelling power is thus diminished ; 

 and if it be diminished in exactly the 

 same decree as the load, the machine 

 will move with its former velocity ; but 

 if it should, at first, be more diminished, 

 the velocity will be less than its lormer 

 velocity, and the balls will again move 

 towards the axis and open the valve, 

 and will, at length, settle into that posi- 

 tion in which the steam admitted to the 

 cylinder is exactly proportioned to the 

 load on the machine ; and the proper 

 velocity will thus be restored. 



By this exquisite contrivance, there- 

 fore, however the load or resistance 

 may from time to time be varied, the 

 velocity will be constantly the same, the 

 impelling power being varied in exactly 

 the same' proportion as the resistance. 



There are various other contrivances 

 for regulating the motion of machinery, 

 amongst which the pendulum of a clock 

 may be mentioned. Our limits preclude 

 us from prosecuting this interesting 

 subject further in this place. We may 

 refer the reader to the second volume of 

 Dr. Brewster's edition of Ferguson's 

 Mechanics for some valuable informa- 

 tion on this subject ; as also to the se- 

 cond volume of Dr. Gregory's Mecha- 

 nics, article *' SCAPEMEXTS." 



CHAPTER XIII. On Mechanical Con- 

 trivances for Modifying Motion. 



(104.) WE have frequently stated, that 

 one of the great ends to be attained by 

 machinery is the change, or modifica- 

 tion of motion. Our impelling power 

 may be rectilinear when a Circular mo- 

 tion may be required in the working 

 point, or the impelling power may be al- 

 ternate, or recipi ocating, when the foice 

 required at the working point is continu- 

 ous. In a word, the motion of the im- 

 pelling power may be of any one species, 

 and that lequiied at the woiking point 

 may be of any other species, li is, tlieie- 

 fore, a very impoitant problem to assign 

 the natme of the machinery which 

 should be interposed between a given 

 impelling power and the working point, 

 so as to produce at that point the etfect 

 required. 



This problem, in its most general 

 form, it is evidently impossible to solve ; 

 nor is it easy to enumerate all the dif- 



ferent motions which the impelling 

 power may have, nor the various 

 motions which these may be requiied 

 to produce at the woi king point. Gene- 

 rally speaking, however, the motions 

 which we meet with in the use of machi- 

 nery may be resolved into rectilinear 

 and circular, that is, one in which the 

 points of the moving part describe pa- 

 rallel straight lines, or one in which 

 they move in circles round a common 

 centre or axis. The-e, again, may be 

 divided into continued and alternate, or 

 reciprocating. That is, the point which 

 moves in a straight line may move con- 

 tinually in the same direction, or it may 

 move backwards and forwards, or up- 

 wards and downwards, its ) ange being 

 limi'ed by two points on the straight 

 line in which it moves. Again, if the 

 point move in a circle, it may move 

 constantly round the centre in the same 

 direction with a continued rotatoiy mo- 

 tion ; or, on the other hand, it may only 

 move over an arc of the circle in one 

 direction, and then i el urn through that 

 aic in the opposite direction, and so on 

 alternately, having a vibrating or reci- 

 procating motion in the circular arc. 



The motions which we have to con- 

 sider may then be resolved into the four 

 following : 



1 . Continued rectilinear motion. 



2. Reciprocating rectilinear motion. 



3. Continued c.rcular motion. 



4. Reciprocating circular motion. 

 We shall devote the present chapter to 

 explain some of the contrivances by 

 which each of these may be modified 

 or converted one into another. 



(105.) To convert continued rectilinear 

 motion in any one direction into con- 

 tinued rectilinear motion in any other 

 direction. 



If the directions of the two motions 

 be in the same plane, this may evidently 

 be effected by a fixed pulley. But if 

 the directions be in difteient planes, two 

 fixed pulleys will be icquisite, one being 

 in the plane of the diiection of one mo- 

 tion, and the other in the plane of the 

 direction of the other motion. 



In Jig. 91, a contrivance is repre- 

 sented, by which a reciiiinear motion in 

 one diiection is made to produce a icc- 

 tilinear motion in a direction at right 

 angles to the former. The inclined 

 plane or wedge A B, is moved under 

 C D, which is^connected by guides with 

 the pillars of the frame. It is evident, 

 that as A B advances from A to- 



