Practical engineering drawing and third angle projection, for students in scientific, technical and manual training schools and for ..draughtsmen .. . By varying the proportions of the links, the point-paths would be correspondingly affected. INSTANTANEOUS CENTR ES. — CENTRO IDS. 53 By tracing the path of a point on P N produced, and as far from N as Z is from P, thestudent will ol>tain an interesting contrast to the Lemniscate. If M and S were joined liy a link and the latter held rigidly in position, it would have beencalled the fixed link; and although its use would not have altered the
Practical engineering drawing and third angle projection, for students in scientific, technical and manual training schools and for ..draughtsmen .. . By varying the proportions of the links, the point-paths would be correspondingly affected. INSTANTANEOUS CENTR ES. — CENTRO IDS. 53 By tracing the path of a point on P N produced, and as far from N as Z is from P, thestudent will ol>tain an interesting contrast to the Lemniscate. If M and S were joined liy a link and the latter held rigidly in position, it would have beencalled the fixed link; and although its use would not have altered the motions illustrated and it isnot essential that it should be drawn, yet in considering a mechanism, as a whole, the line joiningthe fixed centres always exists, in the imagination, as a link of the complete system. INSTAXTAXEOUS CEXTEES. — CEXTROIDS. 159. Let us imagine a al>out to hurl a stone from a sling. Just before he releases ithe runs forward a few steps, as if to add a little extra impetus to the stone. While taking those fewsteps a peculiar shadow is cast on the road l)y the end of the sling, if the day is bright. The Dy. boy moves with respect to the earth; his hand moves in relation to himself, and the end of thesling describes a circle about his hand. The last is the only definite element of the three, jet itis sufficient to simplify otherwise difficult constructions relating to the complex curve which isdescribed relatively to the earth. 54 AND PRACTICAL GRAPHICS. A tangent and a normal to a circle are easily obtained, the former being, as need hardly bestated at this point, perpendicular to the radius at the point of tangency, while the normal simplycoincides in direction with such radius. If the stone were released at any instant it would fly offin a straight line tangent to the circle it was describing about the hand as a centre; but such linewould, at the instant of release, be tangent also to the compound curve. If then we wish a tang
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