dm'- CENTRE OF GRAVITY. 



form of a pyramid is the most durable, because, as it becomes | 



narrower and narrower as it ascends, each stone or brick is \ 

 supported by those below. The pyramids of Egypt, both 



great and small, still remain, and without doubt will do so ; 



for thousands of years to come, while the vast temples are : 



crumbling into ruin. In building, care is taken not to bring ; 



the upper rows of bricks beyond those below, and for this i 



purpose a line and plummet are used. But it does not follow, i 



because a building leans, that the centre of gravity does not I 



fall within the base. There is a high tower at Pisa, a town j 



in Italy, which leans fifteen feet out of a perpendicular di- ^ 

 rection ; strangers tremble to pass by it, still it is found by 



experiment that the line of direction falls within the base^ ^ 



and therefore it will stand while its materials hold together. \ 



The higher the centre of gravity is, the more easily may a 1 



body be overturned. Hence, a wagon or cart with a high ^ 



load is more in danger of being overturned than one with a *j 



heavy load laid lower. This proves the injurious effect of ] 



rising in a coach or boat in danger of oversetting, the centre i 



of gravity being thereby raised, and the line of direction .i 



thrown out of the base. In such circumstances the proper ] 



course is to lie down in the bottom, so as to bring the line of ] 



direction, and consequently the centre of gravity, within the : 



base, and thus remove the danger of oversetting. Rope- ^ 



dancers perform astonishing feats by the assistance of a long ' 

 pole with very weighty pieces of lead at each end, by which 



they balance themselves and recover firm footing, if likely to 1 

 fall on either side. In our ordinary actions we regulate the 



motions of our bodies, as if we were most correctly studying j 



the nature and effects of the centre of gravity. If a man i 

 wishes to rise from a chair, he throws his body forward. If 



he is likely to fall on one side he leans to the other. A cor- ; 



rect knowledge of the centre of gravity in bodies is of the i 

 utmost importance in the science of mechanics, as well as in 



many of the common actions of life. l 



Questions. — 1. What is the centre of gravity ? 2. The line of \ 

 direction? 3. When does a body stand most firmly? 4. Why is a 



pyramid the most durable form of building ? 5. What occasions a  



body to be easily overturned ? C. What is the proper course when a ; 



coach or boat is in danger of oversetting ? 7. On what principle do ^ 



we regulate our ordinary actions ? 8. Show by fig. 12. the comrnon ^ 



centre of gravity of two bodies. 9. Illustrate by fig. 4. the overturning ; 

 of a body, when the line of direction falls out of the base. 



