PREVENTION OF DISEASES AND ACCIDENTS. 



The r?ason of this will perhaps more plainly 

 appear from the following explanations : The 

 Centre of gravity is that point of a body about 

 which all its parts are in equilibria, or balance 

 each other ; and consequently, if this point be 

 supported, the whole body will be at rest, and 

 cannot fall. An imaginary line drawn from the 

 centre of gravity of any body towards the centre 

 of the earth is called the line of direction. Bo 

 dies stand with firmness upon their bases, when 

 this line falls within the base ; but if the line of 

 direction falls without the base, the body will be 

 overturned. Thus, the inclining body ABCD, 

 whose centre of gravity is E, stands firmly on 

 its base CDKF, because the line of direction 

 EM falls within the base. But if a weight, as 

 ABGH, be laid upon the top of the body, the 

 centre of gravity of the whole body and weight 

 together is raised up to I ; and then as the line 

 of direction ED falls without the base at D, the 

 centre of gravity I is not supported, and the 

 whole body and weight must tumble down to 

 gether. 



for three hundred years. But were an additional 

 erection, of any considerable elevation, to be 

 placed upon its top, it would undoubtedly soon 

 tumble into ruins. 



To a somewhat similar cause, in combination 

 with heedlessness and ignorance, may be as 

 cribed many of those accidents which so fre 

 quently happen at spinning mills and other 

 pieces of machinery, by which legs and arms 

 are torn asunder, and the human frame some 

 times mangled and destroyed. 



Fatal accidents have like wise happened from 

 ignorance of the effects produced by the refraction 

 of light. It is a well-known optical fact, that 

 when a ray of light passes from air into water, 

 and is again refracted, the sine of the angle of 

 incidence is in proportion to the sine of the angle 

 of refraction as four to three. From this cir 

 cumstance it happens, that pools and rivers ap 

 pear shallower than they really are their chan 

 nels, when viewed from their brink, being appa 

 rently higher than their true position, in the 

 proportion of three to four ; so that a river eight 

 feet deep will appear from its bank to be only 

 six. This fact, may be at any time perceived in 

 a tub or pail full of water, where the bottom of 

 the vessel will obviously appear to be raised a 

 considerable space above its true position, and 

 its apparent depth consequently diminished. In 

 consequence of this optical illusion, which is not 

 generally known, many a traveller as well as 

 many a schoolboy has lost his life, by supposing 

 the bottom of a clear river to bo within his 

 depth, as, when he stands on the bank, the bot 

 tom will appear one-fourth nearer the surface 

 than it really is. 



This will appear evident from the follow 

 ing illustrations : If a ray of light AC passes 

 obliquely from air into water, instead of continu 

 ing its course in the direct line CB, it takes the 



The tower of Pisa, in Italy, leans sixteen feet 

 -ut of the perpendicular, so that strangers are 

 afraid to pass under it ; but as the plummet or 

 tine of direction falls within its base or foundation, 

 J* ; ; Jo^.^rr_n: :/ *-_!_i_i__ . j: ---ionCH,and approaches the perpendicular 



It is in no danger of falling, if its materials keep 

 together ; and hence it has stood in this state 



PP, in such a manner, that the angle of refrac 



