As you lower yourself toward the ground, your body should move in one motion and should always form a **straight-line** all the way down.

Lower your body toward the floor, maintaining your body in a **straight-line** from head to heel.

There are other arrangements of bars giving **straight-line** motions, and these arrangements together with ths general properties of mechanisms of this kind are discussed in How to Draw a Straight Line by A.

The combination of the parallelogram with a **straight-line** motion, for guiding one of the points in a straight line, is illustrated in Watts parallel motion for steam-engines.

The latter gives values of the constants a and b for different samples of iron and steel, some of which are shown in the following table :- K=a+bH For most samples of steel the **straight-line** law was found to hold approximately up to H=3; in the case of iron and of soft steel the approximation was less close.