xx INTRODUCTION TO MECHANICS. 



pole with an equal weight fastened at each end of it, you would hold it in 

 the middle of the rod, in order that the weights should balance each other ; 

 whilst if it had unequal weights at each end, you would hold it nearest 

 the greater weight, in order to make them balance each other ; and if one 

 were very considerably larger than the other, the centre of gravity would 

 be thrown out of the rod into the heaviest weight (fig. 19). 



SECTTON III. On the Mechanical Powers. 



WE will now proceed to examine the mechanical powers. They are six 

 in number, one or more of which enters into the composition of every 

 machine. The lever, the pulley, the wheel and axle, the inclined plane, 

 the wedge, and the screw. 



In order to understand the power of a machine, there are four things to 

 be considered. 1st. The power that acts : this consists in the effort of 

 men or horses, of weights, springs, steam, &c. 



2dly. The resistance which is to be overcome by the power; this is 

 generally a weight to be moved. The effect of the power, acting in 

 the manner in which in each particular case it is applied, must always 

 be superior to the resistance, otherwise the machine could not be put 

 in motion. For instance, were the resistance of a carriage equal to 

 the strength of the horses employed to draw it, they would not be able to 

 make it move. 



3dly. We are to consider the centre of motion, or, as it is termed in 

 mechanics, the fulcrum, which means a prop ; this is the point about 

 which all the parts of the body move : and, lastly, the respective velocities 

 of the power, and of the resistance. 



We shall first examine the power of the lever. The lever is an in- 

 flexible rod or beam, that is to say, one which will not bend in any direc- 

 tion. For instance, the steel rod to which a pair of scales is suspended is 

 a lever, and the point by which it is suspended, called the prop or fulcrum, 

 is also the centre of motion. The two parts of a lever divided by the 

 fulcrum are called its arms. Now, both scales being empty they are of 

 the same weight, and consequently balance each other (Jig. 20). We 

 have stated that when two bodies of equal weight were fastened together 

 the centre of gravity would be in the middle of the line that connected 

 them; the centre of gravity of the scales must, therefore, be in the middle 

 between them, as the fulcrum is, and, this being supported, the scales 

 balance each other. 



You recollect, that if a body be suspended by that point in which the 

 centre of gravity is situated, it will remain at rest in any position indif- 

 ferently ; which is not the case with this pair of scales, for when we hold 

 them inclined, they instantly regain their equilibrium ; the reason of this 

 is, that the centre of suspension, instead of exactly coinciding with that 

 of gravity, is a little above it ; if, therefore, the equilibrium of the scales 

 be disturbed, the centre of gravity moves in a small circle round the 

 point of suspension, and is therefore forced to rise, and the instant it is 

 restored to liberty it descends and resumes its situation immediately below 

 the point of suspension, when the equilibrium is restored. It is this 

 property which renders the balance so accurate an instrument for weighing 

 goods. If the scales contain different weights, the centre of gravity will 

 be removed towards the scale which is heaviest, and being no longer 

 supported the heaviest scale will descend. The fulcrum of the balance 

 is moveable ; the lever may be taken off the prop and fastened on in 



