18 Mr. \V. R. Birt on a rcmarhahle Barometric Depressio7i. 



The aniline salt, corresponding to the latter, however exists, 

 but could as yet not be obtained in a state of purity. 



All the compounds of aniline with phosphoric acid are 

 anhydrous, like the other salts of this base which have been 

 examined by Professor Hofmann. In this respect they differ 

 from the corresponding soda salts, which nearly all contain a 

 larger or smaller amount of water of crystallization. The two 

 ammonia salts of phosphoric acid which we know, are how- 

 ever likewise anhydrous. 



From the results of the preceding investigation we may 

 conclude that the organic bases comport themselves also with 

 polybasic acids like mineral oxides. I intend, however, to 

 analyse the phosphates of some other organic alkaloids, par- 

 ticularly of those which occur in nature, and are frequently 

 employed in medicine. 



VI. On the remarkable Barometric Depression of the 25tk 

 December 1821. Bj/ W. R. Birt. 



To the Editors of the Philosophical Magazine atid Journal. 

 Gentlemen, 



ON the 26th of November last the barometric depression, 

 noticed at the close of my letter on the Great Symmetri- 

 cal Barometric Wave, returned, making the sixteenth instance 

 in eighteen years. '^I'lie valueof the depression at this station 

 was 29*268 (unreduced): the observation was taken at 10 

 minutes past 8 in the morning. The value is about a tenth 

 higher than the depression at Dublin on the 26th of Novem- 

 ber 1843. In that year the barometer commenced a decided 

 rise on the 27th, which it also did at this station in the pre- 

 sent year. 



My friend E. W. Brayley, Jan., Esq., has suggested that 

 tliis depression is somewhat allied to the great depression 

 which occurred on the 25th of December 1821, and which has 

 been made the subject of an investigation by Prof. Brandes of 

 Breslau, and since by Prof. Dove of Berlin. The reader will 

 fmd Prof. Brandes' paper in the Annals of Philosophy, N. S. 

 vol. iv. p. 263, and Prof. Dove's in the American Journal of 

 Science and Art, vol. xliv. p. 319. A paper on the same 

 subject by Luke Howard is inserted in the Philosophical 

 Transactions, vol. cxii. p. 113. 



There can be no (juestion that the depression of the latter end 

 of Noveniber is periodical : the following interesting question 

 consequently suggests itself. Is the dejjression of the 25th of 

 December also periodical? To answer this question I selected 

 till the readings given in Howard's CUniate of London from 



