Proceedings of the Polytechnic Association. 1055 



exhaled from their hmgs and skin. From the same cause the cold 

 windows of a crowded lecture room, are constantly covered with 

 minute drops of water, which soon collect together and run down the 

 glass in streams. 



The highest point of the thermometer at which vapor begins to 

 be deposited bj the air, is called the dew-jyomt ; it is the point at 

 which dew begins to form. 



5th. The absolute amount expressed in grains of water contained 

 in a cubic foot of air, at the temperature, and relati^'e humidity, as 

 shown by the instrument. 



6th. The depth in inches of water that would be deposited upon 

 the earth by the condensation of all the vajDor held in the atraospliere. 



A discussion of the subject followed the reading of the j)aper, 

 during which several kinds of hygrometers were described, and the 

 imperfection of all commented upon. It was remarked during the 

 discussion that the hygrometer and barometer, taken conjointly, 

 would constitute pretty safe means of foretelling the weather. 



The imperfections of barometers were also discussed. Dr. Bradley 

 asserting that the aneroid barometer at great heights was unreliable, 

 no two of them ao;reeino;. 



Mr. Stetson remarked that a barometer which would record the 

 condition of the atmosphere at different times during the day, in 

 such a manner that easy reference could be made thereto at any sub- 

 sequent time of the day, is a great desideratum, and then he asked if 

 there was any self-registering hygrometer now in use. 



Dr. Tillman replied in the affirmative. 



Mr. Emery remarked that Edson's instrument is certainly very 

 ingenious as regards its mechanical arrangement. He then made a 

 diagram of Prof. Hough's self-registering barometer. 



Dr. Tillman. — Hough's barometer records the changes and prints 

 them. A full description was published in the last volume of the 

 Transactions of the American Institute for 1865-6, pp. 460, 473. 



Dr. Bradley referred to the aneroid barometer, and stated that it 

 could not be depended upon at great elevations. 



Mr. Johnson thinks that the barometer and hygrometer should be 

 used together. To this proposition there was a pretty general agree- 

 ment. 



Dr. Edwards referred to the use made of the barometer by seamen 

 for the purpose of foretelling storms. The British admiralty cause 



