_Mrs. B._ And the advantage gained is proportional to this difference. The most common example that we have of levers of the second kind, is in the doors of our apartments. _Emily._ The hinges represent the fulcrum, our hands the power applied to the other end of the lever; but where is the weight to be moved? _Mrs. B._ The door is the weight, which in this example occupies the whole of the space between the power and the fulcrum. Nut crackers are double levers of this kind: the hinge is the fulcrum, the nut the resistance, and the hands the power. In levers of the third kind (fig. 8.) the fulcrum is again at one extremity, the weight or resistance at the other, and the power is applied between the fulcrum and the resistance. _Emily._ The fulcrum, the weight, or the power, then, each in its turn, occupies some part of the lever between its extremities. But in this third kind of lever, the weight being farther than the power from the centre of motion, the difficulty of raising it seems increased rather than diminished. _Mrs. B._ That is very true; a lever of this kind is therefore never used, unless absolutely necessary, as is the case in raising a ladder in order to place it against a wall; the man who raises it cannot place his hands on the upper part of the ladder, the power, therefore, is necessarily placed much nearer to the fulcrum than to the weight. _Caroline._ Yes, the hands are the power, the ground the fulcrum, and the upper part of the ladder the weight. _Mrs. B._ Nature employs this kind of lever in the structure of the human frame. In lifting a weight with the hand, the lower part of the arm becomes a lever of the third kind; the elbow is the fulcrum, the muscles of the fleshy part of the arm, the power; and as these are nearer to the elbow than to the hand, it is necessary that their power should exceed the weight to be raised. _Emily._ Is it not surprising that nature should have furnished us with such disadvantageous levers? _Mrs. B._ The disadvantage, in respect to power, is more than counterbalanced by the convenience resulting from this structure of the arm; and it is that no doubt which is best adapted to enable it to perform its various functions. There is one rule which applies to every lever, which is this: In order to produce an equilibrium, the power must bear the same proportion to the weight, as the length of the shorter arm does to that of the longer; as was shown by Emily with the weights of 1 _lb._ and of 3 _lb._ Fig. 3. plate 4. We have dwelt so long on the lever, that we must reserve the examination of the other mechanical powers, to our next interview. Questions 1. (Pg. 54) How many mechanical powers are there, and what are they named? 2. (Pg. 54) What is a mechanical power defined to be? 3. (Pg. 54) What four particulars must be observed? 4. (Pg. 54) Upon what will the velocities depend? 5. (Pg. 55) What is a lever? 6. (Pg. 55) Give a familiar example.