502 Proceedings of Royal Society of Edinburgh. 
zeal and perseverance. One little scientific bye-product of extreme 
interest I cannot refrain from quoting. Referring to a hermeti- 
cally sealed glass tube under tests for strength to resist great water 
pressure, “ I enclosed the glass tube in a tube of stout brass, 
“ closed at the bottom only, but was surprised to find that it was 
“ crushed almost flat on the first trial [when the glass tube broke]. 
“ This was evidently due to the fact that water is compressible, 
“ and therefore the relaxation of pressure (produced by the break- 
“ ing of the glass tube) takes time to travel from the inside to the 
“outside of the brass tube; so that for about l/10000th of a 
“ second that tube was exposed to a pressure of four or five tons 
“ weight per square inch on its outer surface, and no pressure on 
“ the inner. The impulsive pressure on the bottom of the tube 
“ projected it upwards so that it stuck in the tallow which fills 
“ the hollow of the steel plug. Even a piece of gun-barrel, which 
“ I substituted for the brass tube, was cracked, and an iron disc, 
“ tightly screwed into the bottom of it to close it, was blown in. 
“ I have since used a portion of a thicker gun-barrel, and have had 
“ the end welded in. But I feel sure that an impulsive pressure 
“ of ten or twelve tons weight would seriously damage even this. 
“ These remarks seem to be of interest on several grounds, for they 
“ not only explain the crushing of the open copper cases of those 
“ of the Challenger thermometers which gave way at the bottom 
“ of the sea, but they also give a hint explanatory of the very 
“ remarkable effects of dynamite and other explosives when fired 
“ in the open air. (It is easy to see that, ceteris jparibus, the 
“ effects of this impulsive pressure will be greater in a large 
“ apparatus than in a small one).” 
In a communication on “Charcoal Vacua ” to the Royal Society 
of Edinburgh of July 5, 1875, imperfectly reported in Nature of 
July 15 of that year, the true dynamical explanation of one of the 
most interesting and suggestive of all the scientific wonders of the 
nineteenth century, Crookes’ radiometer, was clearly given. The 
phenomenon to be explained is that in highly rarefied air a disc 
of pith or cork or other substance of small thermal conductivity, 
blackened on one side, and illuminated by light on all sides, even 
the cool light of a wholly clouded sky, experiences a steady 
measurable pressure on the blackened side. Many naturalists, I 
