1876.] Refraction of Sound by the Atmosphere. 165 



to ascertain how far the refraction of sound caused by the upward variation 

 of temperature may be the cause of the difference in the distances to 

 which sounds of the same intensity may be heard at different times. 



Some rockets, capable of rising 1000 feet and then exploding a cartridge 

 containing 12 oz. of powder, having been procured, an effort was made 

 to compare the distance at which the rockets could be heard with that at 

 which a gun, firing \ lb. of powder and making a louder report than the 

 rockets, could be heard under the same conditions of the atmosphere. In 

 the first instance the rockets and the gun were fired from a spot in 

 Suffolk around which the country is tolerably flat, observers being 

 stationed at different distances. Owing, however, to the effect of the 

 wind and the time required for the observers to proceed to the distant 

 stations, these experiments were not successful in establishing the com- 

 parative merits of the gun and the rockets. They were, however, 

 important as showing that on hot calm days in July the reports of the 

 rockets never failed to be distinctly audible at distances of 4 and 5 miles, 

 although the sun at the time was shining with full force on the ground 

 and rendering the air near the surface so heterogeneous that distant 

 objects seen through it appeared to wave about and twinkle. 



The next attempt was made during a cruise on the east coast. After 

 three weeks cold and windy weather, the 19th of August was a fine day ; 

 and some experiments were made in Lynn Deeps, which revealed a very 

 extraordinary state of the atmosphere as regards the transmission of 

 sound. A party rowed away from the yacht in one of her boats, it 

 having been arranged beforehand that either a rocket or a large pistol 

 was to be fired from the yacht when signalled for ; also that when those 

 on the yacht heard those in the boat call they should answer. The boat 

 proceeded to a distance of 5 miles, until those on the yacht had com- 

 pletely lost sight of it ; but all the time the calls from the boat were 

 distinctly heard by those on the yacht, although after they had lost sight 

 of the boat they ceased to answer the calls. On the boat also not only 

 were the reports of the pistol and rockets distinctly heard, but every 

 answer from the yacht was heard plainly. The last came after an 

 interval of 35 seconds, which gave the distance 3| miles. Nor was this 

 all ; but guns, and on one occasion the barking of a dog, on the shore 

 8 miles distant were distinctly heard, as were also the paddles of a 

 steamer 15 miles distant. 



The day was perfectly calm, there was no wind, the sky was quite clear, 

 and the sun was shining with great power — conditions which have been 

 described as most favourable to the stoppage of the sound by the hetero- 

 geneity of the atmosphere, and which may also be described as most 

 favourable for great upward refraction. On this day, however, it was 

 observed that all the time distant objects 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 



