Chap. XI.

MECHANICS.

953

□

Fig. 1320.

Fig. 1321.

Continuous rectilinear motion, with a uniform velocity, or which varies according to given laws,may be changed into a continuous rectilinear motion, with a velocityof the same nature as that of the movement which produces it, constantor variable, according to a given law, in the same plane or differentplanes.

An endless cord turning round two fixed pulleys gives the idea ofrectilinear motion, which is nothing more than the circular and conti-nuous movement of a circle whose radius is infinite. To make a rightline pass over the distance ab with a certain velocity, whilst the pointc goes over the same distance with the same velocity, a single pulleymay be employed, or two pulleys, if the motion is to be communicatedin different planes.

The conversion of rectilinear motion from one direction to another,with either increased or diminished velocity, is usually effected bypulleys and blocks.

If it be required that the point a should pass over three or four timesthe space of the point c, combinations of pulleys must be employed.

To transform rectilinear movement into circular, as to make thedistance a b traverse in a straight line, and with a certain velocity,to the point a, whilst the point e traverses the distance cd withthe same rapidity and reciprocally ; this is effected generallyby means of a reversing pulley, e , or by two, if the two pointsare to be moved in different planes. If the point c is required torun through two or three times less space than the point a, it iseffected by means of the muffle generally employed to move heavyweights.

Rectilinear motion is changed into con-tinuous rectilinear by the mules of thespinning jennies. The chariot or carriageB, mounted on four wheels, has placedupon it a system of spindles, which aremade to revolve by its motion ; the poweris applied at d, and the carriage is madeto maintain its parallelism by means oftwo cords, m t, sp, the ends of which,n and u, arc attached to two fixed points,and pass through two pulleys sm, tp. Inlike manner the extremities r and u of thesecond arc attached to two other fixedpoints, and pass through two pulleys placed on the same axisas the first; the points u q and nr being so situated that theyarc perfectly parallel, and the cords equally stretched. Thecarriage B, in its primitive position, is made perpendicular ^to the line uq and nr, and when rightly arranged the perfec- ™tion of the work is easily maintained.

The same problem is performed by the motion of a rulemounted on two channelled or grooved wheels placed near itsextremities, and which are equal in diameter, and with theirplanes truly parallel.

The parallel rule has a variety of forms given to it, but thatmost generally used by engineersor draughtsmen has tw'o wheels,as c and d, fixed upon a rod, andwhich, when rolled either back-wards or forwards, preserves aparallel distance : it was the in-vention of Mr. Dollond at thelatter end of the last century.

To make one line move in aparallel direction to another thereare several methods; the simplerule, made to run upon wheels, orthe double, working by means oftwo bars riveted on them, are the ordinary examples : and Ramsden’s movable sights withthree rules is another variety.

The rule a 6 has a horizontal groove running from one end to the other, in which is engageda small fixed cylinder, but which does not traverse it; d is another rule, made freely to

Fig. 1323.

ij

i i

.c =■

a

b

= '* ss @

Fig. 1324.

Fig.1325.