144? Mr. Alison's Ascent of the Peak of Teneriffe. 



upon my thermometer. Another still stronger proof of the 

 extreme clearness of the atmosphere is, that I observed the 

 moon to be indented like a saw, between the light and obscui'e 

 part, which I suppose was caused by the projection of the en- 

 lightened tops of her mountains upon the part which was de- 

 prived of the sun's light. At first I thought it was some optical 

 illusion, as I had just before been standing before the fire, and 

 was almost blind by the smoke; but repeated observations 

 which I made during the night convinced me that I was right 

 in what I first observed. 



There is another observation which I made that may be 

 worth mentioning. Soon after the sun went down the wind 

 became much louder and had an acuter sound, although the 

 force was very considerably less than in the day-time. It has 

 been observed from the earliest antiquity, that the air becomes 

 more sonorous at night than in the day; but I am not aware 

 that the cause of it is well ascertained. The general opinion, 

 I believe, 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 ascer- 

 tained by Dr. Priestley, that the force of the pulsations of 

 sound depends considerably upon the degree of density or 

 rarefaction of the air ; and I think Captain (now Sir Edward) 

 Parry mentions the surprising distance he was enabled to hear 

 sound during the winter at the North Pole. From frequent 

 observations 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 atmosphere. The increased intensity of sound, 

 when I was on the Peak during the night, could not have 

 been caused by an increased density of the atmosphere ; be- 

 cause instead of becoming colder, it was four or five degrees 

 warmer when the sound of the wind became more sonorous. 

 Humboldt has made a similar remark ; and as my observations 

 fully coincide with his opinion, I 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 

 partial 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 propagated with more rapidity through the denser por- 

 tions 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 different portions 



of 



