NO. 8 



WATER-VAPOR TRANSPARENCY— FOWLE 



II 



into the inner tube, a 22 horse-power rotary blower served to 

 thoroughly stir the air and aqueous vapor. Alternate compartments 

 were supplied with pressure and exhaust. This stirring was neces- 

 sary in order to prevent stratification of the air and the consequent 

 blurring and drift of the image of the lamp. It was not desirable 

 to run the blower during an actual spectrum record because of the 

 tremors communicated to the mirrors and to the galvanometer. 



The amount of water vapor was determined by wet and dry 

 thermometers at the spectroscope, at the mirror shelters and at 

 several places in the tube. These were read while the air in the 

 tube was stirred by the i)lower. Check determinations were made 

 several times by Mr. L. B. Aldrich, who absorbed and weighed in 

 tubes of calcium chloride and phosphorus pentoxide the water vapor 

 from known volumes of air taken from the tube. The following table 

 gives the water per cubic meter as measured by the two methods : 



The amount of vapor in the tube could be increased by the intro- 

 duction of a jet of steam into the blower. But when the vapor pres- 

 sure of the water vapor within the tube differed much from that 

 without an equalization took place so rapidly as to render this pro- 

 cedure useless. 



The following table shows a determination of the amount of 

 water vapor in the large tube just preceding, (a), and after, (b), 

 an observation on the transmissibility of radiation through the vapor 

 in the tube. 



August ii, 1914. Barometer 76.5 cm. 



Amount of vapor in path through tube 



:ii7X20X .0001 

 0.234 cm. ppt. HoO.' 



* For brevity the amount of absorbing vapor will be stated as so much pre- 

 cipitable water, " ppt. H2O," meaning the depth of liquid water, which if 

 ■evaporated into a column of the same cross section, would produce the 

 absorbing layer of vapor. This should not be construed as meaning that the 

 liquid water produces the same amount of absorption as the corresponding 

 -vapor. 



