Human Physiology. 187 



odours act as poisons on the nervous system, and delicate persons 

 should avoid strong and heavy perfumes. The rose cold and 

 the hay fever are violent inflammations of the mucous membrane 

 of the nose, caused by odours in some sensitive constitutions. 



Smell gives us proof of the wonderful tenuity and diffusion 

 of matter in the atmosphere ; and we can see from the disper- 

 sion of matter as odours how the germs of such diseases as 

 small-pox, yellow fever, intermittent fever, typhus, &c.,.may fill 

 the atmosphere without always producing a conscious impres- 

 sion on our senses. 



Hearing differs from touch, taste, and smell, in that the 

 sensation is produced by the vibrations of the atmosphere. A 

 bell rung in a vacuum produces no sound. There is nothing 

 to carry on its vibrations. Sound waves spread every way in 

 the atmosphere, but are stronger, and move more rapidly with 

 the wind than against it. In a rapid train on the railway the 

 engine is nearly noiseless, while the last carriages are in the roar 

 of the noise and echoes of all that have gone before. Sound also 

 rises well, and is clearer in moderately high altitudes. When 

 sound vibrations are confined in tubes, they go to great distances 

 with little loss in intensity. Sound pipes might supply a whole 

 village with music, turned on like gas in every dwelling. Sound 

 is conveyed very rapidly and with great force in water, and by 

 means of cords or woody fibre. A slight scratch on one end 

 of the longest piece of timber can be distinctly heard by plac- 

 ing the ear at the other end. A poker, suspended by a double 

 string with its two ends put in the ears, and then struck, sounds 

 like a church bell. 



Sound is reflected in echoes and can be concentrated by 

 concave reflectors, as it is accidentally in whispering galleries. 

 In the focus of the bellying sails of large ships at sea sounds are 

 sometimes heard from the shore at great distances. Architects 

 have yet to discover the mode of constructing the best shaped 

 halls for public speaking and music. 



