56 Mr. Pricleaux on the Detectiori of minute 



tion (col. 7), which was found by experiment, we deduct it 

 from the double difference (col. 8), we get (the numbers in 

 col. 6, corresponding with) the differences between Apjohn's 

 dry-bulb and dew-point (col. 5) ; and so close is this corre- 

 spondence when the slight differences in the leading numbers 

 (cols. 3 and 9) are taken into the accompt, that we can hardly 

 persuade ourselves that it is accidental. 



Hence Apjohn's experiments fix the dew-point at double 

 the descent of the wet-bulb —the effect of current ; and an 

 upward current, in some proportion to the rate of evaporation, 

 must obtain over an evaporating surface. But Mason's num- 

 ber to be deducted, which takes the effect of current in + , is 

 both deduced from and supported by direct experiments. 



It further follows from the premises, and is apparent in the 

 table (cols. 5, 6, 8, 10), that the mean of their respective dif- 

 ferences between the dry-bulb and dew-point is simply double 

 the difference between the dry and wet-bulb ; a relation pre- 

 viously deduced from numerous experiments by August, 

 Bohnenberger, and others. Berzelius in giving us this informa- 

 tion (Tr. de Chim., viii., art. Hygrometre) has not referred 

 to the works in which these experiments are detailed : if you 

 have the means of hunting them out, and giving such abstracts 

 as will enable British meteorologists to judge of the compara- 

 tive degree of confidence to which they are entitled, it may 

 render an important service to meteorology. The best in- 

 struments are subject to the influence of various circumstances 

 in taking the dew-point ; and the experiments of Apjohn, in 

 which most of these interferences are obviated, are objected 

 to by Dr. Hudson, in a paper immediately accompanying 

 their publication. 



In the mean while it must be remembered that Dr. Ap- 

 john's dew-point is that at which the air, by passing through 

 water, took up moisture, and should be under rather than over 

 saturated, so that it would, of course, not deposit any at that 

 temperature. Dr. Mason's, on the other hand, was that at 

 which a metallic surface became dewed ; the air depositing 

 moisture, and being of course supersaturated. Between 

 these two a few degrees of temperature would intervene; the 

 mean of which, the true point of saturation, would be the 

 same as ahove deduced from the tables. It therefore becomes 

 a question whether this simple deduction may not bring us 

 nearer the truth, on the long run, in the present state of our 

 knowledge on the subject, than more elaborate calculations or 

 even observations with our best instruments, considering the 

 temporary character of such observations, and the influence 

 of circumstances to which they are liable. 



I have been led into these inquiries, in considering the pre- 



