METEOROLOGICAL CORRESPONDENCE 405 



longitudinally. In two days it was broken into more than thirty pieces. The 

 tube was ^-inch diameter and §-inch bore. Now, I am unable to give any reason 

 for the breaking of this tube. Are you aware of its cause? 



Remarks. — This is so far from being an unusual phenomenon, that it has 

 been observed by almost every one who has attempted to clean a barometer tube. 

 It results from imperfect annealing, or, in other words, a too rapid cooling, which 

 has left in a condition of opposing tension the outer and inner surface of the 

 glass. The explanation usually given is as follows : When a tube of glass is 

 heated to redness, and then exposed to air of a low temperature, the outer 

 stratum is suddenly cooled, and its particles brought into a state of equilibrium, 

 while the "inner strata are still in an expanded condition; afterwards, when the 

 latter shrink by gradually cooling, they tend to separate from the former, and 

 thus produce a condition of unstable equilibrium. Hence the slightest scratch 

 in the interior, of which the particles tend to separate, is sufficient to rupture 

 the continuity, and cause the fracture of the glass. 



That such a state of unequal density of the outer and inner strata does actu- 

 ally exist, is shown by the effect produced on a beam of polarized light 

 transmitted through the tube at right angles to its axis; and this affords a ready 

 means of detecting the existence in a piece of glass of defective annealing. 



From Scth L. Andrews, Borneo, MicJiigan. 



January 8, 1858. 



The annual occurrence of shooting stars recalled to my mind a similar phe- 

 nomenon witnessed many years ago, which, if it should not be already recorded, 

 may be of some interest. Upon referring to my journal I found the following 

 entry : 



"Dartmouth Colleob, Hanovkr, N. H., August 10, 1830. — Fair. The 

 aurora borealis is very bright this evening; an arch is extended across the 

 heavens about 5° above the horizon, Avith a dark cloud in the space below. 



"Meteors (shooting stars) are darting across the heavens almost every minute; 

 their apparent size varies from that of a spark to that of the planet Venus." 



My recollection of the phenomenon is that they were more frequent than I 

 have ever seen since. I have in mind the same phenomenon witnessed on the 

 9th and 10th of August, 1838; I was then at the Sandwich Islands. I am not 

 able to find that I recorded it, and it is barely probable that it may have occurred 

 a year or two later. On the first evening the meteors were most numerous and 

 brilliant, some of them equalling Venus in the intensity of light. 



Remarks. — The dark segment under the arch is a general, though not an 

 invariable accompaniment of the aurora. It may in part be due, as has been sug- 

 gested, to contrast with the light of the aurora ; but it cannot be entirely referred 

 to this cause since there frequently remains after the light has ceased to appear, 

 a dark cloud in the place previously occupied by the segment. The phenomenon 

 is evidently connected with the precipitation of the vapor of the atmosphere at the 

 time of the passage through it of the auroral beams. I have in some cases seen 

 a perfectly .dear sky almost instantly covered with a mistiness during a vivid 

 display of the aurora. The same also sometimes occurs when no aurora is 

 visible. 



