207. 



the atmosphere in variable proportions ; the moan average being about 4 volumes of 

 carbonic acid in 10,000 of air. Ammonia and all the exhalations from the earth exist 

 as mixtures in the air, and on the presence or absence of these depends its healthful- 

 ness or otherwise. 



The following remarks are from ' Travels in the Air,' by Mr. James Glaisher, F. K. S., 

 and as giving the results of his practical experience and careful observations 

 made during the numerous balloon ascents by that gentleman, they have considerable 

 value : 



' Every one knows that the jrressurc of the atmosphere is measured by means of the 

 barometer. A column of air extending to its limit, of the same area as the barometer 

 tube, is balanced by the column of mercury in the tube ; and if we weigh the mercury, 

 we know the weight or pressure of the column of atmosphere upon that area. If the 

 area of the barometer tube be one square inch, then this would tell us the .pressure 

 of the atmosphere on one square inch. The length of a column of mercury thus 

 balanced by the atmosphere, near the level of the sea, is usually about 30 inches, 

 and if this be weighed, it will bo found to be nearly 15 Ibs. ; therefore the atmo- 

 spheric pressure on every square inch of surface is about 15 Ibs., just one-half as 

 many pounds as he number of inches which expresses the height of the column of 

 mercury. 



' Now, in ascending into the air, part of the atmosphere is below, and part above : 

 the barometer, therefore, has to balance that which is above only, and will, therefore, 

 read less. 



'At the height of three miles and throe-quarters, the barometer will read about 15 

 inches ; there is, therefore, as much atmosphere above this point as there is below, 

 and the pressure on a square inch is 74 Ibs. 



'At a height of between five and six miles from the earth, the barometer-reading 

 will be about ten inches ; one-third of the whole atmosphere is then above and two- 

 thirds beneath, and the pressure of a square inch is reduced to 5 Ibs. 



' The reading of the barometer varies with the altitude at which it is observed, and 

 indicates, by its increasing or decreasing readings, corresponding changes in the 

 pressure of the atmosphere. 



'At the height of 1 mile the barometer reading is 24'7 inches. 

 2 miles 20'3 



3 



4 



5 



10 



15 



20 



16-7 

 137 

 11-3 



4-2 



1-6 



1-0 less. 



1 By the reading of the barometer in the balloon, the distance from the earth is known ; 

 and ii' the. balloon bo situated above clouds, or in a fog, the reading of the barometer 

 indicates the near approach of the earth, and acts as a warning to the occupants of 

 the car to prepare accordingly. In addition to this temporary use, the readings com- 

 bined with those of temperature enable us to calculate the height of the balloon at 

 every instant at which such readings have been taken. 



' The temperature of the dew-point also deserves a few explanatory words. 



' There is always mixed with the air a certain quantity of water, in the invisible 

 shape of vapour, sometimes more, sometimes less, but there is a definite amount 

 which saturates the air at every temperature, though this amount varies considerably 

 with different temperatures. 



' A cubic foot of air at the temperature of 



30 is saturated with 2 grains of vapour of water. 

 49 4 



70 8 



92i 16 



The capacity of air for moisture, therefore, doubles for every increase of temperature 

 of about 20 degrees. The temperature of the dew-point is the temperature to which 

 air must be reduced in order to become saturated by the water then mixed with it ; 

 or it is that temperature to which any substance, such as the bright bulb of a 

 hygrometer, must be reduced before any of the aqueous vapour present will be 

 deposited as water, and become visible as dew. The temperature at which this first 

 bedewing or dulling of bright surfaces takes place is the temperature of the dew-point 



