190 Miscellanies. 



were merged into each other, but the blue and green appeared pre- 

 dominant though faint. The arc 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 4.97°, 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. Redfield. 



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 

 steam 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 source 

 of danger, by attaching to the upper parts of the boiler an air 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." 



As a just apprehension and estimate of the facts, is of great im- 

 portance in guiding our enquiries on this interesting subject, I am in- 

 duced to state my impression that no fatal accident has occurred to 

 any steam boat in the American waters, which can justly be ascribed 

 to the cause mentioned in the foregoing paragraph. In all the acci- 

 dents which have happened in this quarter, the boilers have been 

 crushed or broken through in the direction which is opposite or con- 

 trary to the pressure of the atmosphere. It appears also, from the 

 best evidence which we can obtain, that the pressure of steam at the 

 time of these accidents, as well as on ordinary occasions, has ex- 

 ceeded that of the atmosphere by a pressure of from seven to seventeen 

 pounds to each square inch of surface, and in some instances by a 

 much greater force. It deserves also to be mentioned, that most of 

 these boilers were furnished with " an air valve opening inwards," 

 for the special purpose of obviating any danger which might be sup-^ 

 posed to arise from such a source. 



