PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 435 



The new Avclls on Sag-inaw bay are said to yield as strong brine as tliose 

 of Onondag-a. 



Dr.- Kicliards remarked that the salt works on Saginaw bay which 

 promised so niuch, are not now in operation. During a late visit to that 

 section of" the country he had made inquiries as to the causes which led to 

 closing of these salt manufactories. He was informed that the high price 

 of labor, and the high price of lumber had effected the change. It was 

 found more profitable now to cut down the immense pine forests around 

 Saginaw bay, and saw the timber for the markets on Lake Michigan, and 

 to supply these markets required uU the manual labor that could be 

 obtained. 



The Dew-point. 



The Chairman said he was about to introduce to the audience Prof. 

 "William Baer of Maryland, who would speak on the influence of the Dew- 

 point. As that gentleman's remarks will be made -on the assumption that 

 tiie subject in its general bearings- is understood, and as there may be some 

 present who have not made themselves familiar with this branch of Mete- 

 orology, he would ptesent a few preliminar}'^ facts. 



The atmosphere has the power of holding a certain amount of the vapor 

 of water, ■which is proportioned to the temperature of the air. The warmer 

 the air, the more vapor it ma}' contain. But for each degree of temperature 

 there is a limit beyond wliich tlie amount of vapor in the air cannot be 

 increased. Tlie air being saturated with moisture at a given temperature 

 cannot be cooled without parting with a portion of this vapor in the form 

 of dew. This point of saturation for a given temperature is called the 

 Dew-point. 



The capacity of air for holding moisture may be seen by the following 

 table: 



A cubic foot of air weighs 538.1 grains 



At 100 degrees Fahrenheit it will hold, of water. , 



90 do do do 



80 do do do 



70 do do do 



60 do do do 



50 do do do 



40 do do do 



32 do do do 



From this table it will be seen that the air at GO degrees will hold a 

 little more than twice as much vapor of water as it will at 32 degrees, the 

 melting point of ice. Air at 80 degrees \vill hold nearly twice as much 

 vapor as air at 60 degrees. Air at 100 degrees will hold nearly twice as 

 much vapor as at 80 degrees, and eight times as much vapor when at 32 

 degrees. 



Estimating by bulk, the vapor contained in a given quantity of air at 32 

 deg. F. is 1-160; ut 59 deg. 1-80; at 86 deg. 1-40; at 113 deg. 1-20. The 

 tension of aqueous vapor computed from the experiments ofKegnault, and 

 expressed in inches of mercury is at 32 deg. Fah. 0.181; at 40 deg. 0.2475; 

 at 50 deg. O.Si'.OT; at 60 deg. 0.5178; at 70 deg. 0.7327; at 80 dog. 1.0227; 

 at 90 deg, 1.4088; at 100 deg. 1.917 inches. By means of a complete table 



