Scientific Intelligence — Meteorology. 375 



the cause of it is well ascertained. The general opinion, -I be- 

 lieve, is, that the air becoming colder, is therefore denser and 

 more susceptible of conveying the sonorous waves. This, to a 

 certain extent, may be correct, as it has been well ascertained 

 by Dr Priestley, tliat the force of the pulsations of sound de- 

 pends considerably upon the degree of density or rai'efaction of 

 the air ; and I think Captain, now Sir Edward Parry, men- 

 tions the surprising distance he was enabled, to hear sound 

 during the winter at the North Pole. From frequent observa- 

 tions which I have made in Teneriffe, I am inclined to attribute 

 the intensity of sound at night to a certain increase of moisture, 

 and to an equability of temperature in the different strata of the 

 atmospliere ; because, instead of becoming colder, it was four or 

 five degrees warmer when the sound of the wind became more so- 

 norous. Humboldt has made a similar remark ; and, as many 

 observations fully coincide with his opinion, 1 beg to quote it. He 

 ascribes the diminution of sound during the day to the presence of 

 the sun, which influences the propagation and intensity of sound, 

 by opposing to them currents of air of different density, and par- 

 tial undulations of the atmosphere, produced by unequal heating 

 of different parts of the ground. In these cases a wave of sound, 

 when it meets two portions of air of different density, is divided 

 into two or more waves, a part of the primitive wave being pro- 

 pagated with more rapidity through the denser portions, than 

 the parts that pass through air of less density. In this way the 

 wave is broken down into different parts, which arrive at the 

 ear at different times. These diflFerent portions of the wave 

 passing again through succeeding portions of the atmosphere of 

 different density, may be so wasted and frittered down, as to be 

 incapable of affecting the tympanum. My observation respect- 

 ing the intensity of sound is not confined to the Peak. At the 

 town of Orotava, situated about two miles from the sea, the 

 noise of the waves in the morning occasionally had a grave low 

 sound : at the same time the air appeared to be particularly 

 dry, and distant objects were very indistinct. Towards the mid- 

 dle of the day, or the beginning of the afternoon, the island of 

 Palma, nearly sixty miles distant, could be seen distinctly ; and 

 the ridge of mountains that surround the valley of Orotava 



