MA .VICS. 



r 



MM'llANirs.- I 



FOBCB: ITS DIRECTION, MAGNITUDE, AN; 



Tun ;iim of these Lessons ia to make evident to ordinary intel- 

 ligent persons, who will take a lilil.- tn.ui.l.-, tin- 

 to treat that subject in a popular way, yet HO that 

 .lor may form accurate notions about it, and be enabled 

 to u]i|>ly it to practice in solving common problems by calcula- 

 \Ve httvo isnch to do, hut all depends on the way of lining 

 it. The reader I desire to have is the intelligent mechanic or 

 the country schoolmaster or pupil-teacher, the young 

 t-tiidi-iit who wantw to learn the science through a book without 

 T. tin- college B.A. or M.A. whose mechanics was made a 

 mess of in his young days, and would be glad, without again 

 to a "coach," evi-n late in liv to learn it. I Hhould not 

 of finding even ladies among my scholars. More faith 

 "!nmld be placed in the average human intellect than commonly 

 i~. It ought to bo possible to teach the sciences of form, and 

 to more persons than usually learn them. 

 These aro the "common things" of life, and a knowledge of the 

 nich regulate them ought to bo within the roach of most 

 people, if only the first principles be properly laid down and ex- 

 i, consequences deduced from them in a simple and natural 

 order, and language used which they can understand. I ask you, 

 tlii-n, to approach the subject without fear. Study simultaneously 

 with these lessons those upon Arithmetic ; for, as we proceed, 

 a knowledge of the four Common Rules of Arithmetic and of 

 Proportion will bo found essential. Any other mathematics you 

 may require, I shall teach 

 you as we go along, but the 

 ojuount will be small. Ob- 

 serve: accurate mechanical 

 conceptions, and the power 

 of solving mechanical pro- 

 blems by construction by 

 rule and compass or calcula- 

 tion, are the obj ects we aim at. 

 First, then, let us ascertain 

 what our science treats of. 

 I believe it may accurately 

 be described as follows : 



MECHANICS is the science 

 of force applied to a material 

 body or bodies. 



This let me fully explain. Mechanics is concerned about force 

 that is its great subject. But it considers it only in the con- 

 sequences which follow its application to a body or bodies which 

 must be material. A force may push through an empty point 

 of space ; but, as it can make no impression on that point, 

 Mechanics does not consider it under such circumstances. The 

 body to which it is applied may bo of any size, even an atom of 

 matter, sometimes termed "a material point;" and Mechanics 

 does inquire what effect forces have on such atoms. But, in 

 the more common problems, it is concerned about bodies of 

 visible and tangible magnitude, such as a block of stone, a 

 beam of timber, a girder of iron, a cannon ball, the earth itself, 

 tho moon, or the sun. 



This being clearly understood and agreed on, our next 

 question is, What is force ? I answer 



FORCK is the power, or agent, whatever bo its nature, by which 

 motion is produced in a body, or a tendency to motion accom- 

 panied by strains or pressures in its parts. 



For instance, a blow is given by tho bat to tho cricket ball, 

 or a bolt is fired from a cannon : the blow in the ono ( 

 the exploding gunpowder in the other, furnish for, 

 of which is the motion of the ball or bolt. Steam ent 

 cylinder of an engine, and away to work goes the machinery 

 connected with it, moving and printing this POPULAR 

 EDUCATOR. Here again is force, tho elasticity of tho steam, 

 and its effect is motion. A stone is let loose at the top of a 

 tower, or from a balloon, and it falls to tho ground : what 

 makes it fall ? The great Earth does, which, by its attraction, 

 pulls the stone towards itself. This attraction is the force 

 producing the stone's motion. And if any of you doubt, or feel 

 any difficulty about this, let him take a magnet and put one of 

 its ends near a few loose iron-filings, scattered over a piece of 

 paper, and he will see how this ia possible. The tilings will 



DIAGRAM ILLUSTRATING THE APPLICATION OF FORCE. 



more toward* the magnet, and stick to it, in the rery MOM 

 that the atone move* to, and stick* to, the earth until 

 penon pulls it away by a stronger face*. And ao likewise does, 

 the electrified ball draw toward* itself the amall piece* of cork 

 or feather we place near it In all these PS.SSS, 700 nee, there is, 

 first, a body, the ball, or bolt, or atone, or iron-filing, or cork ; 

 v, a force u ' ; an<l, thirdly, motion produced, 



t iko now the lamp which hangs from the ceiling, 

 at rest; but the earth, by ito attraction, ia trying to pull it 

 down, and down it would come were we to cot the chain or rod 

 by which it ia suspended. Here, then, ia force again, bat it 

 produces only tendency to motion. But observe further, that 

 although the lamp does not more, the chain that hold* it U 

 strained by its weight And not only ia the chain strained, but 

 so is the ceiling joint to which it ia attached; and, aa thia joist 

 rests its ends on the walla, thia strain ia transmitted to the wall* 

 in the form of pressures on them. There ia thna tendency to 

 motion, Htniin. and pressure produced aa the effect of the force 

 applied by the earth to the lamp, but no motion. And, if any of 

 you feel a difficulty in believing in those strains, let him suppose, 

 instead of tho lamp, a ton weight of iron suspended from the 

 coiling : what will follow ? The chain will snap, or the joist, 

 or even ceiling, will givj way, and down all will come on the 

 floor. They snap or give way because they are *train*d beyond 

 r.-nu'tii. So, in like manner, when a train stands at rest 

 on one of those great iron girder bridges that span our riTers, 

 there is tendency to motion, with strains and pressures; the 

 great Earth below pulls at the train to bring it into the water ; 

 but tho bridge resists, bears the pressure of the weight on it, 



and is strained throughout 

 its length besides. A more 

 familiar instance is the 

 etruggle of two wrestlers. 

 No one will doubt that in 

 tho contest great farce is put 

 forth by each. For a mo- 

 ment they are motionless, 

 like statues ; the forces are 

 balanced, but the strain on 

 their muscles is t. 

 There is in each tendency 

 to motion, caused by the 

 force put forth by the other, 

 but as yet no motion. At 

 last one of the combatants 



prevails ; his force ends in producing motion, and his adversary 

 falls to the ground. 



These examples will, I trust, be sufficient to make clear to 

 you tho account I have given you of force, namely that it ia the 

 agency by which motion ia produced in a material body, or a 

 tendency to motion with pressures or strains. You will now under- 

 stand the reason why Mechanics is divided into two branches, 

 Statics and Dynamics. Statics is the branch which treatn of 

 forces which balance each other, and produce only tendencies 

 to motion with pressures and strains, and is so called from the 

 Latin word sto, which means " to stand," or " be at rest" 

 Forces which thus balance one another are said to be tn equili- 

 bria, a Latin expression which denotes the balancing of equal 

 weights ; and it ia important that yon should keep the expression 

 in memory, as we shall have frequent occasion to use it. The 

 other branch, Dynamics, treats of force or forces which do not 

 balance. ono another, but produce motion, and was so named from 

 the Greek word Swa^ts (du'-na-mis), power, under the mistaken 

 notion that there was more power in force when its effect U 

 motion, than when it produces strain. This, we have seen, ia 

 no. the cose: but the term "Dynamics" may, notwithstanding, 

 continue to be used without leading to error. The two branches 

 we may therefore define or describe as follows : 



STATICS is tho branch of Mechanics in which forces are 

 considered which equilibrate, or balance one another, producing 

 tendencies to motion, with strains and pressures. 



DYNAMICS is the branch of Mechanics in which forces are 

 considered which produce motion. 



Now it so happens that, of these branches, Statics is the 

 simpler and easier, and more natural for the student to 

 commence with. Questions about forces which balance each 

 other are not so complicated as those which involve motion. 

 The reason is, that time enters into all problems of motion, but 



