PNEUMATICS. 291 



narrow end, to receive the lips, and confine the 

 voice of the speaker. 



The beat of a watch may be heard to twice the dis- 

 tance through a speaking-trumpet, that it can be 

 heard at without one. This experiment is said to 

 succeed equally whether the trumpet is cylindrical 

 or conical. See the Edinburgh Encyclopaedia, ar- 

 ticle Acoustics, Part n. Sect. ii. The instrument 

 seems to produce its effect, by preventing the im- 

 dulae generated in the air, from diffusing themselves 

 all round, by which means they are longer subjected 

 to an impulse in the same direction. 



SECT. III. 



AIR, AS THE VEHICLE OF HtEAT AND MOISTURE. 



393. JL HE Earth and the Atmosphere, taken 

 generally, receive at all times nearly the same quan- 

 tity of heat and light from the sun. 



As the whole of one side of the earth is constantly 

 turned to the sun, the only difference in the quan- 

 tity of heat and light which the earth receives in a 

 given time, must arise from the changes which take 

 place in its distance from the sun at different sea- 

 T 2 sons 



