EFFECTS PKODUCED UPON BUILDINGS. 119 



made to oscillate violently whilst C remains still ; or vice 

 versa, C may be caused to oscillate whilst A and B remain 

 still. In the one case the period of shaking will have 

 been synchronous with the natural period of A and B, 

 whilst in the latter it will have been synchronous with 

 that of c. This would seem to show us that if the 

 natural period of vibration of a house, or of parts of it, at 

 any time agree with the period of the shock, it may be 

 readily thrown into a state of oscillation which will be 

 dangerous for its safety. 



Second Experiment. — Bind a and B together with a 

 strip of paper pasted between them. (The paper used 

 was three-eighths of an inch broad and would carry 

 a weight of nearly three pounds.) If the table be now 

 shaken as before, A and B will always have similar move- 

 ments, and tend to remain at the same distance apart, 

 and as a consequence the strip of paper will not be broken. 

 From this experiment it would seem that so long as the 

 different portions of a building have almost the same 

 periods of vibration, there will be little or no strain upon 

 the tie-rods or whatever contrivance may be used in 

 connecting the dififerent parts. 



Third Experiment — Join A and c, or b and C with a 

 strip of paper in a manner similar to the last experiment. 

 If the table be now shaken with a period approximating 

 either to that of A and B, or with that of c, the paper will 

 be suddenly snapped. 



This indicates that if we have different portions of a 

 building of such heights and thicknesses that their natural 

 periods of vibration are different, the strain upon the 

 portions which connect such parts is enormous, and it 

 would seem, as a consequence, that either the vibrators 

 themselves, or else their connections, must, of a necessity, 

 give way. This was very forcibly illustrated in the Yoke- 



