152 Royal Society : — 



April 25. — Major- General Sabine, Treas. and V.P., in the Chair. 



The following communication was read : — 



" On the Distribution of Aqueous Vapour in the Upper Parts of 

 the Atmosphere." By Lieut. -Col. Richard Strachey, F.R.S. 



The experiments of chemists having shown that any gas will flow 

 into a space occupied by another gas, and diffuse itself there as 

 though the space were a vacuum and the second gas not there, it 

 was suggested by Dalton that the atmosphere might be considered 

 to he a combination of as many distinct atmospheres as it has gase- 

 ous components, and that the actions of each of these might be 

 treated of separately, and irrespective of the others. Meteorologists, 

 pursuing this idea, have proposed to separate the pressure of the 

 aqueous vapour from the whole barometric pressure of the atmo- 

 sphere, and thence to infer the pressure of the permanently elastic 

 portion, or as it has been called, the Gaseous Pressure, or the 

 Pressure of the Dry Air. 



It is my object to inquire how r far the facts of the matter will 

 support Dalton's suggestion of the possible independent existence of 

 an atmosphere of aqueous vapour, and whether we can in truth eli- 

 minate the pressure of this vapour by subtracting the observed ten- 

 sion from the total barometric pressure, in the manner that has com- 

 monly been done of late years. 



And first as to the hypothesis of an atmosphere of aqueous vapour 

 pressing only upon itself. If there be such an atmosphere, the 

 general laws of pressure of elastic fluids will apply to it, as they do 

 to the mixed atmosphere. But in consequence of the small specific 

 gravity of the vapour, the rate of the diminution of pressure in the 

 upper strata of the vapour atmosphere w r ould be much slower than in 

 the mixed atmosphere ; and irrespective of any variation in the law 

 of the decrease of temperature, the height to which we should have 

 to ascend in the vapour atmosphere to produce a given diminution of 

 pressure, would be to the corresponding height in the mixed atmo- 

 sphere inversely as the specific gravities of the atmospheres, that is, 

 as 1 to '625 or as 8 to 5. Thus by ascending about 19,000 feet in 

 the atmosphere, the barometric pressure is found to be reduced one- 

 half; and consequently it would be necessary to ascend to about -| of 

 19,000 fe€t, or upwards of 30,000 feci, to produce a corresponding 

 diminution of pressure in a vapour atmosphere, or to reduce the ten- 

 sion, say from 1 inch to -} an inch. 



Now let us compare this result with the observed facts. This is 

 done in the annexed Table I., in which the ratio of the tensions of 

 the vapour at heights extending to 20,000 feet with the surface 

 tension, as actually observed, is set down in juxtaposition with "the 

 ratios that should hold good in an independent vapour atmosphere. 

 We here see that in reality the tension is reduced to one-half of what 

 it is at the earth's surface by an ascent of about 8000 feet, instead 

 of 30,000 feet, as the hypothesis of the independent vapour atmo- 

 sphere would require. 



