746 



MECHANICS, (ANIMAL.) 



piu, which is fixed in a frame called a Hock. The 

 circumference of llie wheel has a groove for a rope to 

 move in. 'Hie pdley is said to be fixed or movable, 

 according as its block admits of motion or not. A 

 fixed pulley gives no mechanical advantage, but it 

 enables us to apply force more conveniently, by 

 changing its direction. A man standing on the deck 

 of a ship is able, by means of one fixed at the top of 

 the mast, to raise a weight to that point by drawing 

 downwards. In the same manner, ore is raised from 

 mines, and water from deep wells. The wheel, in 

 the grooved circumference of which the rope passes, 

 gives facility to its motion by preventing the neces- 

 sity of its bending suddenly round a sharp edge, and 

 diminishes the friction by transferring it from the rope 

 to the axis of the wheel. One or more grooved 

 wheels, called sheaves, set in a block, and moving 

 freely round an iron axis, constitute a pulley, and the 

 combination of pulley and ropes, a tackle. If the 

 rope, instead of being attached to the weight, passes 

 through a movable pulley attached to the weight, 

 and terminates in a hook or ring in the upper block, 

 the tackle becomes an engine by which another ad- 

 vantage is gained. As, in this case, the weight is 

 supported by two parts of a rope, each part sustain- 

 ing one half, the power necessary to support one of 

 these parts, is equal to only one half the weight sup- 

 ported, and, by drawing upon one end of the rope, 

 with a power a little greater than one half of the 

 weight, the whole weight will be raised. It is on this 

 principle that advantage is gained by the pulley. If 

 the weight were supported by the four parts of a 

 rope, which passed through two fixed and two mov- 

 able pulleys, each part sustaining one fourth of the 

 weight, a power equal to one fourth part of the 

 weight, attached to the free end of the rope, would 

 balance the whole weight, and something more than 

 one fourth would raise it. This advantage is pur- 

 chased by the space through which the power must 

 move, and the time occupied by the motion. To 

 raise a weight fifty feet, by the combination last 

 mentioned, the power must move over a space of 

 200 feet. 



The nature of the different kinds of pulley will be 

 better understood by the aid of the following illustra- 

 tion, taken from Grier's Mechanic's Calculator. 



The accompanying engraving exhibits various forms 

 of the pulley. A B is a beam from which they are 

 suspended. 



1234 



A! IB 



No. 1, is the fixed pulley in which there is no other 

 advantage gained than that the power P and weight 

 W move in a contrary direction. No. 2, is a movable 

 pulley, in which the power P by moving upwards 

 raises the pulley, to the block of which the weight W 

 is attached ; but the one end of the string being at- 

 tached to the beam A B, the power must move twice 

 as fast as the weight, and there will be a gain of 

 power proportional. No. 3, is a combination of two 

 movable pulleys, in which the gain of power will be 

 four ; and No. 4 is a combination of two fixed and twe 

 movable pulleys, in which the gain of power will be 

 the same as in No. 3. If in a system of pulleys, 

 where each pulley is embraced by a cord, attached 



at one end to a fixed point, and at the other to II e 

 centre of the movable pulley next above it, and the 

 weight is hung to the lowest pulley ; then the effect 

 of the whole will be = the number 2 multiplied by 

 tself, as many times as there are movable pulleys in 

 i.he system : thus, if there be 4 movable pulleys, then 

 2x2x2x2=16; wherefore, if the weight be 

 one Ib. it will be sustained by a power of one 

 ounce avoirdupois. 



When there are any number of movable pulleys on 

 one block, and as many on a fixed block, the pulleys 

 are called Sheaves, and the system is called a 

 Muffle ; and the weight is to the power inversely as 

 one is to twice the number of movable pulleys in the 

 system. 



In all the above cases of the pulley, the strings, 

 cords, or ropes, are supposed to act parallel to each 

 other ; when this is not the case, the relation of 

 power and weight may be found by applying the 

 principle of the parallelogram of forces ; thus, draw 

 ab in the direction of the power's 

 action and of that length, taken 

 from a scale of equal parts, which 

 expresses the quantity of the 

 power ; next draw bd a perpen- 

 dicular to the horizon, and from a 

 draw ad parallel to be, the direc- 

 tion of the string, which is fastened 

 at c: then the power is to the 

 weight, as ba is to Id; and the strain on the hook 

 at <?, is as ad to dl>,- these lines being all measured 

 on the same scale of equal parts. It may be further 

 observed that the pulley is a species of lever of the 

 second kind ; where the point at which the string is 

 fastened may be called the fulcrum ; the axis of the 

 pulley the place of the weight, and the place of the 

 power the other end of the lever ; or, the diameter 

 of the pulley may be reckoned the length of the lever, 

 the weight being in the middle. 



A great many experiments made by Rondelet, 

 have shown that, for most purposes, the best propor- 

 tions for the wheel of a pulley are, 1. that its diam- 

 eter should be five times its thickness ; 2. that the 

 diameter of the pin should be one-twelfth of that of 

 the wheel ; 3. that the wheel should have about one- 

 twelfth of its thickness on each side for its play in 

 the block. 



Additions might be made to the list of mechanical 

 powers, with as much propriety as some of those 

 enumerated are retained. The engine of oblique 

 action, usually called the toggle joint, might be 

 called a mechanical power. It is, however, more 

 properly, a combination of levers, acting on the prin- 

 ciple of the funicular machine. 



MECHANICS' ANIMAL. Mechanism of the 

 human Skeleton. There is scarcely a part of the 

 animal body, or an action which it performs, or an 

 accident that can befall it, or a piece of professional 

 assistance which can be given to it, that does not 

 furnish illustration of some truth of natural philoso- 

 phy ; but we shall here only touch upon as many 

 particulars as will make the understanding of others 

 easy. 



The cranium, or skull, is an instance of the arched 

 form, answering the purpose of giving strength. The 

 brain, in its nature, is so tender, or susceptible of 

 injury, that slight local pressure disturbs its action. 

 Hence a solid covering, like the skull, was required 

 with those parts made stronger and thinker which 

 are most exposed to injury. An architectural dome 

 is constructed to resist one kind of force only, always 

 acting in one direction, namely, gravity ; and there- 

 fore its strength increases regularly towards the bot- 

 tom, where the weight and horizontal thrust of the. 

 whole are to be resisted ; but, in the skull, the tena- 



