Refraction of Sound by the Atmosphere. 325 



loomed considerably, i. e. appeared lifted. This showed that the air 

 was colder near the surface of the sea than it was above. It is 

 to this circumstance that the extraordinary distances to which 

 sounds were heard on this day is supposed to be due. The dimi- 

 nution in the temperature of the air being downwards, the sound, 

 instead of being lifted as it usually is, was brought down, and 

 thus intensified at the surface of the water, which, beiug perfectly 

 smooth, was thus converted into a sort of whispering-gallery. 



The report of the pistol and the sound of the voices were attended 

 with echoes, but not so the reports of the rockets ; and it is sug- 

 gested that these so-called echoes may be found only to attend 

 sounds having a greater intensity in one direction than in another. 



The second part of the paper refers to a phenomenon noticed by 

 Arago in his report of the celebrated experiments on the velocity 

 of sound made on the nights of the 21st and 22nd of June, 1822. 



It was then found that, although the guns fired at Montlhery 

 could be distinctly heard at Yillejuif (11 miles distant), those fired 

 at Yillejuif could not be heard at Montlhery without great atten- 

 tion, and at times (particularly on the second night) they were 

 not heard at all — although on both nights the wind was blowing 

 from Yillejuif to Montlhery, the speed of the wind, which was very 

 light, being about 1 foot per second. No explanation of this phe- 

 nomenon was offered by the observers, although it was much com- 

 mented on. And on the second night the gun at Yillejuif, which on 

 the previous night had been pointed upward, was brought down in 

 the hope that this might improve its audibility (this step, however, 

 was found to render matters worse than before). 



IrOm this lowering of the gun at Yillejuif it seemed as though 

 there was probably some difference in the conditions under which 

 the guns at the two stations were placed, as if that at Yillejuif 

 was fired from a level, while that at Montlhery might be fired over 

 a parapet. An inspection of the district confirmed this view ; for 

 Yillejuif is on a low flat hill, while Montlhery is on the top of a 

 steep cone ; and not only is it 80 feet above Yillejuif, but it is sur- 

 mounted by the mound of an old castle, which is supported by a 

 vertical wall towards Yillejuif and surrounded by a low rampart. 

 Hence it is suggested that in all probability the advantage of the 

 gun at Montlhery was due to its being fired over this parapet, while 

 that at Yillejuif was fired from the level ground. 



The fact that the wind blowing from Yillejuif did not reverse 

 this advantage, suggested the possibility that at night, when the di- 

 minution of temperature is downward, a light wind may not pro- 

 duce the same effect upon sound as when the diminution of tem- 

 perature is upward, as it generally is during the day. 



To ascertain if this is the case, some observations were made on 

 some calm nights in May and June of the present year, from which 

 it was found : — 



(1) That when the sky was cloudy and there was no dew, the sound 

 of an electric bell 1 foot above the grass could always be heard fur- 

 ther with the wind than against it ; but 



(2) that when the sky was clear and there was a heavy dew, the 



