TRANSACTIONS OF TUB SECTIONS. S3 



been of an intense yellow dashed with green, became diffused, and 

 threw off luminous portions which passed the zenith. 



Notice of an Instrument to observe minute Changes of Terrestrial 

 Magnetism. By W. Ettrick. 

 The needle, suspended in a glass case by a single fibre or hair, 

 bears a graduated card, which is observed by a telescope properly 

 adjusted at right angles to its surface. 



Notice of a new Rubber for an Electrical Machine . By W. Ettrick. 



On a new Method of Investigating the Specific Heats of Gases. By 

 James Apjohn, M.D., M.R.I. A., Professor of Chemistry in the 

 Royal College of Surgeons, Ireland, 



In the commencement of this communication, which was made orally 

 in the Physical Section, Dr. Apjohn drew attention to some prior re- 

 searches of his on the same subject, which he had explained at the 

 meeting of the British Association held in Dublin. Having established 

 (see Notices of Communications made at the Dublin Meeting, p. 27.) 



48 ad T) ^ ' 

 that the formula/" =/'— x ^— includes the solution of the well- 



^ '^^ , . e 30 



known dew-point problem, it follows that a = (/'—/') x 7^-7 X — , 



■which expression, when the air is perfectly dry, or, what amounts to 



the same, when f"=o, becomes a ^ -—-r x — . Hence, if/' and d be 



48 a p 



determined by observation, that is, if the temperature of air t, and 

 the stationary temperature of a wet thermometer immersed in the 

 same medium, first brought to a state of perfect desiccation, be ob- 

 served, the specific heat of air may be calculated. This formula also, 

 as is obvious, is equally true of the other gases, that is, when applied 

 to similar observations made upon them, it will give their relative 

 specific heats under equal volumes ; and such results, it is scarcely 

 necessary to say, when divided by the specific gravities, will give the 

 specific heats under equal weights. Such, as has been already fully 

 explained, was the principle of the method which he had first adopted. 

 The numbers, however, in the last column of the table published by 

 the British Association, (see Notices of Communications made at the 

 Dublin Meeting, p. 32,) are not, as they are represented to be, the 

 specific heats of equal Aveights, but of equal volumes, for the divi- 

 sion by the specific gravities had, through hurry, been omitted. Nor 



'f'andfare the respective forces of vapour at the dew-point <", and at/', the 

 stationary temperature of the wet thermometer : d is the depression of tem- 

 perature shown by the latter instrument, e the caloric of elasticity of aqueous 

 vapour at (', a the specific heat of air, ;; the existing, and 30 the mean 

 pressure. 



VOL. v. — 1836. D 



