129 
1917-18] Sound-Waves of the East London Explosion. 
than that of the sound waves, and this suggests that they travelled along 
a less elevated path. 
The relative horizontal velocity of the sound-waves and air- waves offers 
further evidence on this point. Within a few miles of the source of sound 
the sound-waves outran the others, both then travelling near the surface of 
the ground. As they approached the boundary of the inner area, this order 
was generally reversed, though the evidence is somewhat conflicting, and 
the reversal was maintained all over the outer area. The earlier arrival of 
the air- waves at some distance from the source is probably due to the 
sound-waves following a higher and colder route. 
At the narrow western end of the silent zone there are two places 
(Uppingham and Lilford) at which the sound may have been heard. 
In this region, also, are concentrated 9 out of 16 observations on the 
shaking of windows and the disturbance of pheasants in the silent zone. If 
both sound-waves and air-waves travelled at a low altitude over this zone, 
but the air- waves at a less height than the sound-waves, these observations 
would be explained by the diffraction of those portions of the waves which 
were nearest the ground. 
The time-observations as a rule are given too roughly to be of much 
service in determining the velocity of the sound-waves. The best estimates 
of the time to the nearest minute are 6.51 p.m. at the Royal Observatory, 
Greenwich (for which I am indebted to the courtesy of the Astronomer- 
Royal), 6.59J at Little Bytham (near Grantham), and 7.0 at Spalding and 
Swineshead (near Boston). The times at Little Bytham and Spalding were 
given by signalmen on duty ; that at Swineshead by the stationmaster, 
who was on the railway platform. The distances of these places from the 
source of sound being respectively 89, 89, and 100 miles, it follows that, 
taking the above times as exact, the mean velocities to the three places 
would be 921, 870, and 978 feet per second. Thus, allowing for some 
inaccuracy in the time-records, it would seem that the mean horizontal 
velocity of the sound-waves was less than its normal value for the 
temperature then prevailing on the ground. 
I conclude, then, (i) that both sound-waves and inaudible air-waves 
crossed the silent zone along paths at no great height above the ground ; 
(ii) that the path of the air-waves lay at a less height than the path of the 
sound-waves ; (iii) that the paths of both waves crossed the narrow western 
end of the silent zone at lower altitudes than elsewhere ; and (iv) that the 
observation of the inaudible air- waves beyond the limits of the sound-areas 
was due to the low and more nearly horizontal path by which they travelled. 
VOL. XXXVIII. 
(Issued separately June 17 , 1918 .) 
9 
