190 Miscellanies. 
were merged into each other, but the blue and green appeared pre- 
dominant though faint. The are was seen for nearly a quarter of 
an hour, and then disappeared but an instant before the parhelia. 
During the succeeding night and day there was a heavy fall of snow 
and the thermometer fell to 0. During the succeeding week, the 
mean temperature was 4.979, the highest being 15° and the lowest 
—11°; affording, in my view, a confirmation of the theory of halos, 
parhelia, &c. given in the Library of Useful knowledge, No. 19. 
3. On the supposed collapse of steam boilers, and the means of pre- 
venting explosions; by W. C. RepriELp. 
TO THE EDITOR. 
Dear Sir—The review of Professor Renwick’s Treatise on the 
Steam Engine, which appeared in the last number of your valuable 
Journal, contains the following passage. 
“ A great proportion of the fatal accidents which have occurred in 
stean boats, have arisen from a collapsing of the boilers; that is, in 
consequence of the sudden formation of a vacuum in the boiler, by 
which means the sides of the boiler have been crushed together by 
external pressure, and the hot water and steam forced out with great 
violence. It seems a very easy matter to provide against this soute? 
of danger, by attaching to the upper parts of the boiler an alr valve 
opening inwards. Whenever the tension of the steam becomes less 
than the pressure of the atmosphere, the valve will open and restore 
the equilibrium.” 
Asa just apprehension and estimate of the facts, 1s of great i 
portance in guiding our enquiries on this interesting subject, Lon 
duced to state my impression that no fatal accident has occurred 1 
any steam boat in the American waters, which can justly be ascribe 
to the cause mentioned in the feregoing paragraph. In all the acer 
dents which have happened in this quarter, the boilers have bees 
crushed or broken through in the direction which 1s opposite or col 
trary to the pressure of the atmosphere. It appears also, from 
best evidence which we can obtain, that the pressure of steam at 
time of these accidents, as well as on ordinary occasions; has 
ceeded that of the atmosphere by-a pressure of from seven to sevent 
pounds to each square inch of surface, and in some instances by * 
much greater force. It deserves also to be mentioned, that most ri 
these boilers were furnished with “an air valve opening inwa 
for the special purpose of obviating any danger which might be on: 
posed to arise from such a source. f 
reat im 
