508 
NATURE 
[JuLy 8, 1915 



Bird Migration. 
A SMALL item which may be of interest to those 
watching bird migration was noted on a recent voyage 
to America. 
On s.s. St. Louis, of the American Line, about 
6.45 p.m. (ship time) on May 3 a swallow came 
on board, evidently very tired, white breast feathers 
rather dirty, and, settling down, was caught 
by one of the passengers. It took some water but 
died during the night. There was no identification 
band on either leg. 
The point of interest is that the ship was then about 
492° N. lat. and 232° W. long., which would put her 
some 560 miles west of Cape Clear—about W. by S.— 
and some 680 miles N.W. from Cape. Jn inisterre. The 
wind had been fairly steady from E.S.E. for the pre- 
vious thirty hours, blowing 20-30 miles an hour. 
The first swallows had been noticed at the place 
where I now am on Saturday, May 1, and no doubt 
the bird which reached the St. Louis had got separated 
from the general migration. 
The bird seemed in fairly 
good condition, its brown 
throat and indigo head were sleek and glossy; the 
soiled breast feathers may have been due to the 
smoke as the bird came in from the lee side. 
Ep. WILDING. 
Antrim, June 23. 
steamer’s 
Dunedin, Jordanstown, Co. 
Mercury Ripples showing Interference. 
THE accompanying print of a photograph of mercury 
ripples showing interference, made in this laboratory, 
exhibits singularly well the circular waves from the 
two sources as well as the interference pattern pro- 
duced. The two points of disturbance are maintained 

by a forked pointer attached to the prong of a fork 
of frequency 50. With daylight illumination from a 
window, and a rotating sector to render the effect 
stroboscopic, a good natural picture oe _the surface is 
obtained. Se STARLING. 
Physical Laboratory, Municipal Technical Institute, 
West H: ami, E. 
Man’s True Thermal Environment. 
agree with Mr. Grabham (Nature, June 
as to the unsuitability of the constant- 
(37° C.) psuchrainometer for many parts 
surface. It is for this very reason, 
combined with the advantage of its much greater 
simplicity, that I am experimenting with the con- 
stant-energy form of instrument mentioned at the end 
of my former letter (NaTtuRE, May 6, p. 260). 
The effect of moisture can be brought into play 
with any type of psuchrainometer by providing its 
NO. 2384, VOL. 95] 
I FULLY 
24, Pp. 451) 
temperature 
of the earth’s 

exposed surface with a water-wetted muslin cover, 
and no doubt in this condition the apparatus approxi- 
mates more closely to the human body. 
My interest in the matter, however, is physical 
rather than physiological. My immediate aim is to 
study the extent to which “atmospheric cooling” can 
be predicted from the readings of the existing meteoro- 
logical instruments. It seems best, therefore, to begin 
with the simplest case of cooling, namely, that w hich 
is free from the thermal complications accompanying 
evaporation. James Roperr MILNE. 
Physical Laboratory, University of Edinburgh, 
July 1. 
‘ 


HIGH EXPLOSIVES. 
R. LLOYD GEORGE’S recent speech in 
the House of Commons, as Minister af 
Munitions, emphasised the very important part 
played by high explosives in the present war. It 
is essential at the outset to distinguish clearly 
between a propellent charge, which forces the 
projectile or shell through the bore of the gua, 
and the high explosive charge filling the shell 
itself and causing it to burst, through the inter- 
vention of a time or percussion fuse. Modern 
military propellants consist of gelatinised gun- 
cotton (nitro-cellulose), either alone or mixed with 
varying proportions of nitro-glycerine, pressed into 
any required shape. The finished explosive is of 
a colloidal, horny nature, and a piece of it held 
in the fingers, w rhilst burning, can be blown out 
quite easily. A charge lit in the enclosed space 
of the chamber of a gun can discharge a projec- 
tile with a velocity of about 1000 yards per second, 
developing in the chamber a pressure of, perhaps, 
twenty tons on the square inch. If the same 
quantity of the ungelatinised material were ignited 
in the gun-chamber it would detonate and blow 
the gun to pieces. 
There is thus a wide difference between the 
effects produced by the burning of a propellant in 
the open, and in the chamber of the gun. In 
the latter case, before the projectile begins to 
move, the gases evolved produce pressure in the 
chamber, thus greatly accelerating the velocity of 
the explosive reaction. 
The forces at work in the gun, however, are 
insignificant in comparison with those brought 
into play when a high explosive detonates. Even 
in the open, without any containing envelope 
other than a thin cylinder of paper, the writer 
has obtained with a high explosive a velocity of 
detonation of the explosion wave of some seven 
miles per second. When the high explosive is 
in an enclosed space, such as a shell, the velocity 
of the detonation wave is greatly accelerated, and 
in an almost infinitesimal period of time the ex- 
plosive is converted into gases. The volume of 
gases produced varies according to the nature of 
the explosive, but, generally, for those used in 
shells, it may be taken that, at the temperature 
of explosion, the volume of gas evolved occupies 
from 15,000 to 20,000 times the volume of the 
original explosive. This is the reason for the 
enormous destructive and shattering effect of a 
high explosive. 


