2.56 Dr. Hudson's Method for inferring the Dew-point 



course of the Euphrates in that navigator's (Nearchus's) time, 

 was only 206^ miles, whilst in the present day it is as great 

 as about 400, it seems to me we have no alternative but to at- 

 tribute the difference between these two measurements to the 

 gain of the land upon the sea during the intervening period 

 of 2160 years." Surely 206\ miles from a town on anyroad 

 or river is plain enough. Nearchus then sailed on the gulf 

 up to about the present town of Simauvu, where he found 

 Diridotis, and about the present town of Duffas on the Ti- 

 gris*, Alexander the Great entered its mouths. We have 

 now merely to explain his sailing down the Euleus from Susa, 

 the navigation across the canal, and the localities of these nar- 

 rations. Though, indeed, our labours do not even termi- 

 nate here, for " the possibility has been hinted to Col. Ches- 

 ney, that the actual site of Babylon may be some 30 or 40 

 miles north-west of Hiliah," a discovery which would of 

 course require the shifting about and readjustment (if indeed 

 the matter has been thought of) of the historical correspond- 

 ing landmarks. But it is now, I believe, becoming evident, 

 that the incongruity of all this with ttje plain facts of history, 

 is beginning to be felt. Such a theory may appear feasible 

 enough while kept within a circle of eloquent generalities; 

 drawn thence for a closer view and application to circum- 

 stances, we find it not formed for the occasion. 



Temple Chambers, July 22, 1835. W. G. Carter. 



XXXI. Proposed Method for inferring the Dew-point from 

 the Indications of the Wet-bulb Hygrometer. By Henry 

 Hudson, M.D., M.R.LA.f 



A S the expansion of air by heat is uniform, and equal to 

 -^*- T ^o tn °f * ts volume (at 32°) for each degree above that 

 temperature, the relative volumes at different temperatures 



will (cceteris paribus) be proportional to 448 + O and, of 



course, the relative densities will be inversely as 448+^ : hence 

 putting f for the elastic force of (or pressure on) air, and t' 

 for any temperature, we have density at 212° (under 30 pres- 

 sure): density at 212° under/' : : 30 \f f \ also density at 212° 



that the channels of the Tigris are clearer on account of the greater ra- 

 pidity of their current down the declivity towards the Euphrates, whose 

 waters could not run in such channels without running up a slope. I have 

 before pointed out, that this declivity distinctly negatives the joint delta 

 of two long separated streams. 



• See Col. Chesney's Map in Report. 



f Communicated by the Author. 



