Pendolointerrotto - Public domain geometry drawing

Imagine that this page represents a vertical wall, with a nail driven into it, and from

the nail let there be suspended a lead bullet of one or two ounces by means of a �ne

vertical thread, AB, say from four to six feet long. Draw on this wall a horizontal

line DC, at right angles to the vertical thread AB, which hangs about two inches in

front of the wall. Now bring the thread AB with the attached ball into the position

AC and set it free; first it will be observed to descend along the arc CBD, to pass

the point B, and to travel along the arc BD, till it almost reaches the horizontal

CD, a slight shortage being caused by the resistance of the air and the string; from

this we may rightly infer that the ball in its descent through the arc CB acquired

an impetus on reaching B, that was just sufficient to carry it through a similar arc

BD to the same height. Having repeated this experiment many times, let us now

drive a nail into the wall close to the perpendicular AB, say at E or F, so that

it projects out some five or six inches in order that the thread, carrying the bullet

through the arc CB, may strike upon the nail E when the bullet reaches B, and thus

compel it to traverse the arc BG, described about E as center. From this we can see

what can be done by the same impetus that previously starting at the same point

B carried the same body through the arc BD to the horizontal CD. Now you will

observe that the ball swings to the point G in the horizontal, and you would see the

same thing happen if the obstacle were placed at some lower point, say at F, about

which the ball would describe the arc BI, the rise of the ball always terminating

exactly on the line CD. But when the nail is placed so low that the remainder of

the thread below it will not reach to the height CD (which would happen if the nail

were placed nearer B than to the intersection of AB with the horizontal CD) then

the thread leaps over the nail and twists itself about it.

1638

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