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arising from any source, might cause the destruction of the 

 edifice. 



Before entering fully into this, it is necessary to con- 

 sider the effect of impact, that is, of a body or weight 

 falling with the accelerated force due to gravitation. The 

 effect of a falling body impinging upon another, is in the 

 ratio of the weight and the distance fallen through. Thus 

 a weight of 1 lb. falling through three feet of space, 

 would produce the like effect as a weight of 3 lbs. 

 falling through one foot. I have not found any accurate 

 statement of the space through which a body must fall, 

 to double by impact the effect of its dead weight; but I 

 suspect from the results of rough experiments, that such 

 space is only about three quarters of an inch; I find also 

 that in experimenting with any contrivance like a scale 

 beam, there is much less of force from flexure or extension, 

 and much uncertainty arising from vibration. I, however, 

 further find, that a weight of | of a lb. falling through 

 thirty inches, on a wire which would support 30 lbs. 

 weight, just snaps off the wire. 



It would, therefore, appear that where a structure is ex- 

 posed to the impact of bodies falling even from slight 

 distances, the load on it must be very light in proportion 

 to the breaking weight, in order to allow for the effect of 

 impact. It has, however, been observed by Mr. Hodgkin- 

 son, that a beam or a wire, when moderately loaded, is 

 better able to sustain impact than when without load, up 

 to a certain ratio between the breaking weight, the weight 

 sustained, and the weight of the body impinging; that is, 

 the greatest resistance to impact is when the weight of 

 the load, including that of the impinging body, is equal 

 to one-third of the breaking weight of the beam or wire. 



An idea is entertained by some persons, that no material, 

 however strong, is capable of sustaining any weight, how- 



