456 THE POPULAR SCIENCE MONTHLY. 



of his voyage to the polar regions. He says, " I found no difficulty, 

 in that cold and quiet ah*, in conversing with a man a mile away." It 

 will be noticed that in both of these cases the air was mechanically homo- 

 geneous ; that is, there were no alternating currents of hot and cold air. 



In striking contrast with these may be mentioned the condition of 

 the air as a vehicle for sound in the burned district of Boston, just after 

 the fire had swept over it. There were many places, where there was a 

 mixture of hot air, smoke, steam, and currents of cold air, in which the 

 shouts of two people hardly a hundred feet apart, although audible, 

 were so confused and indistinct as to make communication entirely 

 impossible, and this too in quiet parts quite remote from the scene of 

 conflagration. This effect was noticed when there were mingled cur- 

 rents of hot and cold air. That is, the air was mechanically heteroge- 

 neous. Humboldt speaks of the great difference in transmissive power 

 for sound of the tropical air during the day and at night, and attrib- 

 utes this difference to the homogeneous condition of the air at night 

 as compared to its heterogeneous condition in the day-time, due to 

 convection-currents rising from the heated sands. 



A large number of instances might be cited, and we should find 

 that a clear homogeneous air transmits sounds readily, while an atmos- 

 phere broken by alternating convection-currents of warm and cold 

 air is a very poor vehicle for sound. The explanation of this is not 

 difficult. The original ray of sound, striking upon the first current of 

 air, is partially reflected and partially transmitted. 



The loss of the reflected portion causes a decrease in the intensity 

 of sound. The transmitted portion, striking upon a second current, is 

 likewise divided, and its transmitted portion continues to be so divided 

 as many times as there are variations in the density of the air. Its 

 reflected portion, as well as that of all the succeeding reflections, in- 

 stead of being wholly lost, is interrupted in its backward course by 

 the first current of air, and reflected along the path of the primary 

 wave, but following it at an interval of time, depending upon the 

 thickness of the current of air. Each reflection being thus again and 

 again divided and reflected, we have, following close upon the primary 

 wave, a multitude of secondary waves, which, falling later and later 

 upon the ear, greatly mask the distinctness of the original sound, and 

 give rise to indistinctness and confusion. 



It is evident, then, that in order to procure the proper propagation 

 of sound, one must do away with these air-currents. It must be re- 

 membered, however, that, when large numbers of people are crowded 

 into halls, the air within is usually subjected to very considerable dis- 

 turbances in order to obtain even indifferent ventilation. Registers, 

 sprinkled here and there over the floor, send up their currents of hot 

 and cold air. Opened windows or other cold-air ventilators send in 

 their currents of cold air. If these currents could be made visible to 

 the eye, as they can be detected by proper apparatus, we should see 



