73 8 THE POPULAR SCIENCE MONTHLY. 



a colossal tuning-fork, the prongs of which were placed between the 

 poles of a powerful electro-magnet. This fork, which interrupted 

 the battery current, at the proper time, by its own motion, was 

 able to put a heavy cord, thirty feet in length, in the most ener- 

 getic vibration, and for an indefinite time. I propose, at the present 

 time, to speak of those sympathetic vibrations which are pitched so 

 low as not to come within the limits of human ears, but which are 

 felt rather than heard, and to show how they may be seen as well 

 as felt. 



All structures, large or small, simple or complex, have a definite 

 rate of vibration, depending on their materials, size, and shape, and as 

 fixed as the fundamental note of a musical cord. They may also vi- 

 brate in parts, as the cord does, and thus be capable of various increas- 

 ing rates of vibration, which constitute their harmonics. If one body 

 vibrates, all others in the neighborhood will respond, if the rate of 

 vibration in the first agrees with their own principal or secondary 

 rates of vibration, even when no more substantial bond than the air 

 unites a body with its neighbors. In this way, mechanical disturb- 

 ances, harmless in their origin, assume a troublesome and perhaps a 

 dangerous character, when they enter bodies all too ready to move at 

 the required rate, and sometimes beyond the sphere of their stability. 



When the bridge at Colebrooke Dale (the first iron bridge in the 

 world) was building, a fiddler came along and said to the workmen 

 that he could fiddle their bridge down. The builders thought this 

 boast a fiddle-de-dee, and invited the itinerant musician to fiddle away 

 to his heart's content. One note after another was struck upon the 

 strings until one was found with which the bridge was in sympathy. 

 When the bridge began to shake violently, the incredulous workmen 

 were alarmed at the unexpected result, and ordered the fiddler to stop. 



At one time, considerable annoyance was experienced in one of 

 the mills in Lowell, because the walls of the building and the floors 

 were violently shaken by the machinery: so much so that, on certain 

 days, a pail of water would be nearly emptied of its contents, while on 

 other days all was quiet. Upon investigation it appeared that the 

 building shook in response to the motion of the machinery only when 

 that moved at a particular rate, coinciding with one of the harmonics 

 of the structure ; and the simple remedy for the trouble consisted in 

 making the machinery move at a little more or a little less speed, so 

 as to put it out of time with the building. 



We can easily believe that, in many cases, these violent vibrations 

 will loosen the cement and derange the parts of a building, so that it 

 may afterward fall under the pressure of a weight which otherwise 

 it was fully able to bear, and at a time, possibly, when the machinery 

 is not in motion ; and this may have something to do with such acci- 

 dents as that which happened to the Pemberton Mills in Lawrence. 

 Large trees are uprooted in powerful gales, because the wind comes in 



