Csclllation of tJte Barometer. 317 



it is indeed almost self-evident, that, within a very moderate ele- 

 vation above the surface of the earth, the humidity as well as the 

 temperature of the air, cease to be affected by the daily variation. 

 To argue, then, upon the variations of humidity at the surface 

 of the earth as if they extended equally throughout the atmo- 

 sphere, as is done in the prevalent theory, is certainly erroneous. 



8. But it remains to be considered how far the daily variation 

 of humidity, such as it really exists, is capable of affecting the 

 barometer. This we may easily estimate, uot by using such a 

 complex mode of measurement as the hygrometer, but by simply 

 observing what weight of water enters or quits the atmosphere 

 during the various periods of the day. For it is evident that if 

 a cubic foot of water, instead of resting on the surface of the 

 earth, evaporate and form a part of the atmosphere, the weight 

 of the latter will be increased by the precise weight of the cubic 

 foot of water. Siuiilarly, if a stratum of water a foot deep eva- 

 porate all over the world, the pressure of the atmosphere in every 

 place will increase by an amount represented by one loot depth 

 of water, which is equal to a column of mercury 88 hundredths of 

 an inch in depth, since mercury is 13*G times as dense as water. 

 In short, the evaporation of a foot of water will cause the baro- 

 meter to rise about nine-tenths of an inch. The precipitation 

 of a foot depth of rain, supjiosing it to extend equally over all 

 parts of the globe, must occasion a corresponding fall of the 

 barometer. 



Now applying these considerations to one jiarticular subject, 

 namely the semidiurnal variation of the barometer, I argue that 

 the state of the hygrometer is a matter of jjcrfect indifference, 

 and that the evaporation of water by day and its deposition as 

 dew at night, or as rain at occasional times, are the only modes 

 in which the barometer will be appreciably affected through the 

 agency of humidity*. Thus, in a place where the evaporation is 

 30 inches in the year, or "082 inch in each day on the average, 

 the barometer cannot be affected thereby to a greater extent on 

 an average during each day than '006 inch (or •082-i-13'6) ; and 

 if, with Dalton, we estimate the yearly depth of dew at 5 inches, 

 its average daily effect on the barometer will be only 001 inch. 

 These estimations perhaps apply roughly to England, and are 

 the total amounts which we have at our disposal to explain a 

 daily oscillation of the barometer, there amounting to about 

 •030 inch, even supposing that evaporation takes place wholly 

 in such a manner as to assist the prevalent theory. 



* A(|ucoii.s v.Tpoiir, wliilc it so remains, expands and rontracts by heat 

 and cold like any other }^as, and may thurelorc have a trifling effect upon 

 the barometer in the siame manner us the gaseous atmospiierc. 



