A third advantage which often attends the use of tools and machines is that force may be made to produce motion at various distances, in various directions, and in various degrees of velocity. Thus as to distance, a man standing on the ground can raise a weight to the top of a house by a pulley. So, also, a water-wheel may by the connections of machinery produce motion at considerable distances from it. Then as to direction, horizontal motion may be converted into vertical, rotary into straight, etc. The velocity of motion is generally varied by cog-wheels. Thus a wheel of 60 cogs revolving once in a minute, playing on a wheel of 10 cogs, will make it revolve once in 6 seconds. [Illustration: Fig. 181.] Another advantage of tools and machines is that they secure a better mode of applying power than we otherwise could have. Thus when several men are pulling on a rope much power is lost by their pulling irregularly, a difficulty which is removed by the pulley. The same can be said of applying pressure by the screw. One man presses more steadily, and therefore more effectually, than fifty men would without the screw. The arrangements of tools and machines are so made as to provide convenient ways of applying our strength. An instrument, for example, for moving a weight by hand is so shaped as to hold the weight well, and also to afford a good handle for the hand to grasp. The common claw hammer is a very good illustration. We grasp the nail by an iron claw, with the handle we can apply not merely the force of the hand, but that of the whole arm, and then we have the immense lever power of the instrument. We have a good illustration of convenience in an instrument, in what is called a Lewis, represented in Fig. 181. It is used for raising blocks of stone in building. It has three parts, A B C. It is used in this way: A hole is made in the upper part of the block of stone to be raised in shape like the instrument; then A and C are inserted, and B is pushed in between them. With the ring, D, bolted through the instrument the stone is raised to its place by the ordinary machinery. The principle of the instrument, you see, is that of the wedge.