SOUND. 



falls to one-half its usual elevation, and where therefore the air 

 is highly rarefied, sounds are greatly diminished in intensity. 

 Persons who ascend in balloons find it necessary to speak with 

 much greater exertion, and, as would be said, louder, in order to 

 render themselves audible. When Saussure ascended Mont 

 Blanc, he found that the report of a pistol was not louder than a 

 common cracker. 



13. Violent winds and .other atmospheric agitations affect the 

 transmission of sound. When a strong wind blows from the 

 hearer towards the sounding body, a sound often ceases to be 

 heard which would be distinctly audible in a calm. A tranquil 

 and frosty atmosphere placed over a smooth and level surface is 

 favourable to the transmission of sound. Lieutenant Forster held 

 a conversation with a person on the opposite side of the harbour of 

 Port Bowen, in the third polar expedition of Sir Edward Parry, 

 the distance between the speakers being more than a mile. 



It is said that the sound of the cannon at the battle of Waterloo 

 was heard at Dover, and that the cannon in naval engagements 

 in the Channel have been heard in the centre of England. 



14. Liquids are also capable of propagating sound. Divers 

 can render themselves audible at the surface of the water ; and 

 stones or other objects struck together at the bottom produce a 

 sound audible at the surface. 



It appears from the experiments of M. Colladon, made at 

 Geneva, that sounds are transmitted through water to great 

 distances with greater force than through air. A blow struck 

 under the water of the Lake of Geneva was distinctly heard 

 across the whole breadth of the lake, a distance of nine miles. 



Solid bodies, such as walls or buildings interposed between the 

 sounding body and the hearer, dimmish the loudness of the 

 sound, but do not obstruct it when the sound is made in air ; but 

 it appears from the experiments of M. Colladon, that the inter- 

 position of such obstacles almost destroys the transmission of 

 sound in water. 



1 5. Solids which possess elasticity have likewise the power of 

 propagating sound. If the end of a beam composed of any solid 

 possessing elasticity be lightly scratched or rubbed , the sound will 

 be distinct to an ear placed at the other end, although the same 

 sound would not be audible to the ear of the person who produces 

 it, and who is contiguous to the place of its origin. 



The earth itself conducts sound, so as to render it sensible to 

 the ear when the air fails to do so. It is well known, that the 

 approach of a troop of horse can be heard at a distance by putting 

 the ear to the ground. In volcanic countries, it is said that the 

 rumbling noise which is usually the prognostic of an eruption is 

 190 



