SOUND. 61 



of the bulb should be at least thirty or forty times larger than 

 that of the tube ; so that the mass of mercury may be as 

 little affected as possible whilst it rises and falls ; for the 

 height of the column is taken from the surface of the mer- 

 cury in the bulb to its height in the tube. 



Questions. — 1. What is the construction of the barometer ? 2. 

 Upon what does the height of tlie mercury depend ? 3. Why is the 

 air heaviest in dry weather ? 4. Why does it feel heaviest in damp 

 weather ? 5. How may the height of a mountain be ascertained by 

 the barometer? 6. What is indicated by the convexity and concavity 

 of the mercury ? 7. Upon what other construction are barometere 

 made than that first described ? 



LESSON 29. 



Sound. 



Humid'ity, moisture. The degrees of moisture in the air are 

 measured by an instrument called a Hygrom'eter, of which 

 there are various kinds ; whatever contracts or expands by 

 the moisture or dryness of the atmosphere is capable of being 

 formed into one. 



Sound arises from a tremulous or vibrating motion in elas- 

 tic bodies, which is caused by a stroke or collision, and is 

 carried to the ear through the medium of the air. The pro- 

 duction of sound therefore depends upon three circumstan- 

 ces, a sonorous body to give the impression, a medium to 

 convey it, and the ear to receive it. Sonorous bodies, how- 

 ever, are merely the instruments by which a peculiar species 

 of motion is communicated to the air. It is true that when you 

 ring a bell, both the bell and the air are concerned in the pro- 

 duction of sound : but sound, strictly speaking, is a perception 

 excited in the mind by the motion of the air on the nerves 

 of the ear ; the air, therefore, as well as the sonorous bodies 

 which put it in motion, is only the cause of soimd, — the im- 

 mediate effect is produced by the sense of hearing : I5r 

 without this sense, there would be no sound. The vibrating 

 air strikes the ear, and causes in the mind the perception of 

 sound. 



If you endeavour to ring a small bell, after you have sus- 

 pended it under the receiver in an air-pump, from which 

 the air has been exhausted, no sound will be produced. By 

 6 



