Natural History. 223 



the horizon, appeared as the light of a building on fire appears at 

 night, at the distance of several miles. I expected, in that instance, 

 every hour to hear that some building in Shutesbury, or New 

 Salem, had been burnt, and, so strong was my belief of it, that I 

 repeatedly asked my neighbours whether they had heard of any 

 such event. At last, I met a gentleman who had just come from 

 one of those towns, who told me he had heard of no fire from that 

 quarter, which convinced me the phenomenon was merely atmos- 

 pheric." — SiUimans Journal^ xii. 380. 



3. On the Determination of the Mean Temperature of the Air. — 

 This subject has been investigated by M. G. G. Hallstroem, who 

 gives the following algebraic formula, which correctly represents the 

 mean temperature for all Europe. 



V = i(,xf+xe) - 0.33 + 0.41 sin. [(ti - 1) SO-' + 124° 8')]' 



V = mean temperature. 

 n rr: the ordinal number of the month for which the temperature is 



to be calculated (thus, for March, n = 3). 

 i (^y + -J^ ^) = the mean temperature taken as the mean of ob- 

 servations taken at ten o'clock in the morning and evening. 

 In winter i {j^ f + x e) = v very nearly ; whilst, in summer, this 

 quantity is f of a degree greater than v at Paris, Halle, and Abo. — 

 Annul. derPhys, und Chem. 1825, p. 373. 



4. Indelible Writing. — As the art of mail can unmake whatever 

 the art of man can make, we have no right to expect an indelible 

 ink: however, a sort of approximation to it may be made as fol- 

 lows ; — Let a saturated solution of indigo and madder in boiling 

 water be made, in such proportions as give a purple tint ; add to 

 it from one sixth to one eighth of its weight of sulphuric acid, ac- 

 cording to the thickness and strength of the paper to be used : this 

 makes an ink which flows pretty freely from the pen, and when 

 writing, which has been executed with it, is exposed to a consi- 

 derable, but gradual, heat from the fire, it becomes completely 

 black, the letters being burnt in and charred by the action of the 

 sulphuric acid. If the acid has not been used in sufficient quantity 

 to destroy the texture of the paper, and reduce it to the state of 

 tindery the colour may be discharged by the oxymuriatic and oxalic 

 acidsy and their compounds^ though not without great difficulty. 

 "When the full proportion of acid has been employed, a little crump- 

 ling and rubbing of the paper reduces the carbonaceous matter of 

 the letters to powder ; but by putting a black ground behind them, 

 they may be preserved, and thus a species of indelible writing is 

 procured, (for the letters are, in a manner, stamped out of the paper,) 

 which might be useful for some purposes, perhaps for the signature 

 of bank-notes. 



5. Peculiar Crystals of Quartz. — Mr. W. Phillips has met with 

 some remarkable crystals of quartz, which occurred imbedded in the 



