470 Prof. Apjohn on a Formula for ascertaining 



of the Aldborough corals ; but surely there are few who, from 

 such limited data, would consider themselves qualified to dis- 

 pute the accuracy of results obtained from the more exten- 

 sive investigation of others. I could bring forward consider- 

 able evidence (in addition to that which has already appeared) 

 for the purpose of showing that the red crag is not of diluvial 

 origin, and that the Itamsholt deposit is a part of the coral- 

 line stratum. It appears to me, however, that the reliance 

 Mr. Woodward places on the testimony of others does not ex- 

 tend beyond such facts as fall within the sphere of his own ob- 

 servations also : any additional facts that I might adduce in 

 support of my previous views, being new to him, might there- 

 fore be regarded as fabrications. 



W. M. Higgins, Esq., F.G.S., has in his possession a ma- 

 nuscript paper by Mr. R. C. Taylor, in which is pointed out 

 the separation between the lower and upper beds of crag, and 

 to which I shall more particularly allude in a future commu- 

 nication. 



I am, Gentlemen, yours, &c. 

 Guy's Hospital, Nov. 16, 1835. Edw. Charlesworth. 



LXI. Experimental Investigation of a Formula for infer- 

 ring the Dew-point from the Indications of the Wet-bulb 

 Hygrometer, By James Apjohn, M.D., Professor of Che- 

 mistry in the Royal College of Surgeons, Ireland. 



[Continued from p. 274, and concluded.] 



'T^HE most direct method of testing our formula consists, 

 -*■ as has been already observed, in comparing its results 

 with dew-points experimentally determined. In order, how- 

 ever, that this criterion be decisive, it is not only necessary 

 that the depressions be considerable in amount, but also, as 

 is obvious, that the dew-points be accurately known. Now, 

 the registers to which I have had access do not perfectly sa- 

 tisfy either of these conditions, the depressions being ge- 

 nerally small, and the observations made with an instrument, 

 DanielPs hygrometer, the difficulty of observing with which 

 is universally admitted. In reflecting on this matter it oc- 

 curred to me that both difficulties might be evaded in the fol- 

 lowing simple manner. Let air saturated with moisture, and 

 whose temperature is, therefore, necessarily its dew-point, be 

 heated, and let the temperature of the heated air be taken, as 

 also that shown by a moist-bulb hygrometer subjected to the 

 action of a current of it. Let then, by the application of the 

 formula, the dew-point belonging to the two latter observa- 



