SYMPATHETIC VIBRATIONS IN MACHINERY. 739 



gusts ; and, if these gusts happen to be timed in accordance with the 

 natural swing of the tree, the effect is irresistible. The slow vibra- 

 tions which proceed from the largest pipes of a large organ, and 

 which are below the range of musical sounds, are able to shake the 

 walls and floors of a building so as to be felt, if not heard, thereby 

 furnishing a background of noise on which the true musical sounds 

 may be projected. - 



We have here the reason of the rule observed by marching ar- 

 mies when they cross a bridge ; viz., to stop the music, break step, 

 and open column, lest the measured cadence of a condensed mass of 

 men should urge the bridge to vibrate beyond its sphere of cohesion. 

 A neglect of this rule has led to serious accidents. The Broughton 

 bridge, near Manchester, gave way beneath the measured tread of 

 only sixty men who were marching over it. The celebrated engineer, 

 Robert Stephenson, has remarked ' that there is not so much danger 

 to a bridge, when it is crowded with men or cattle, or if cavalry are 

 passing over it, as when men go over it in marching order. A 

 chain-bridge crosses the river Dordogne on the road to Bordeaux. 

 One of the Stephensons passed over it in 1845, and was so much struck 

 with its defects, although it had been recently erected, that he noti- 

 fied the authorities in regard to them. A few years afterward it 

 gave way when troops were marching over it. a 



A few years ago, a terrible disaster befell a battalion of French 

 infantry, while crossing the suspension-bridge at Angers, in France. 

 Reiterated warnings were given to the troops to break into sections, 

 as is usually done. But the rain was falling heavily, and, in the hurry 

 of the moment, the orders were disregarded. The bridge, which was 

 only twelve years old, and which had been repaired the year before at 

 a cost of $7,000, fell, and 280 dead bodies were found, besides many 

 who were wounded. Among the killed or drowned were the chief of 

 battalion and four other officers. Many of the guns were bent double, 

 and one musket pierced completely through the body of a soldier. 

 The wholesale slaughter at the bridge of Beresina, in Russia, when 

 Napoleon was retreating from Moscow, in 1812, and his troops crowded 

 upon the bridge and broke it, furnishes a fitting parallel to this great 

 calamity. 



"When Galileo set a pendulum in strong vibration by blowing on it 

 whenever it was moving away from his mouth, he gave a good illus- 

 tration of the way in which small but regularly-repeated disturbances 

 grow into consequence. Tyndall tells us that the Swiss muleteers tie 

 up the bells of the mules, for fear that the tinkle should bring an 

 avalanche down. The breaking of a drinking-glass by the human 

 voice, when its fundamental note is sounded, is a well-authenticated 

 feat ; and Chladni mentions an innkeeper who frequently repeated the 



1 Edinburgh Philosophical Journal, vol. v., p. 255. 

 * Smiles's " Life of Stephenson," p. 390. 



