PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 425 



sensitive. In the Government vessels they take a note of the barometer 

 every half hour, bnt if the change is sudden it is reduced to fifteen min- 

 utes, or even less if the fall is very sudden. 



The Chairman. — The aneroid barometer is very useful and convenient for 

 the farmer in foretelling the changes in the weather. It is very generally 

 supposed that whenever the index shows a diminished pressure of the 

 atmosphere there will be rain; this does not always follow. Wlien the 

 pressure is very low, strong winds succeed though there may be no rain. 

 But it is lowest for wind and rain together, the simplest rule is that when 

 the fall is sudden there will be a storm, 



Mr. Bartlett. — There seems to me to be one paradox in the barometer, 

 and that is, when the air is dr}'' the mercury rises, showing an increase of 

 pressure; but when the air is largely saturated with moisture, and would 

 appear to be very heavy, the mercury falls, 



Dr, Rowell.— If the barometer is at thirty inches of course there will be 

 more moisture in the atmosphere, because there is an inch more of air, but 

 the air can only be saturated with water equal to its own density, and can- 

 not sustain anything heavier than itself; for when the atmosphere becomes 

 saturated beyond its own density the moisture falls to the earth, 



Mr. Garvey. — If that were the case would it be possible to get a mixture 

 of alcohol and water, as alcohol is lighter than water, or could we get a 

 mixture of carbonic acid with the air? We know that moisture will rise 

 from water into the atmosphere, but by what process? we know it is not 

 by gravitation; the mixture of alcohol and water can be made to diffuse 

 itself equably. 



Hygrometer. 



Dr. Rowell explained the wet-bulb hygrometer on the table. He said 

 this instrument consists of two delica.te thermometers placed side bj' side. 

 The bulb of one is covered with cotton, leading into a vessel containing 

 water. The capillary attraction through the cotton keeps the bulb always 

 moist, the temperature of which becomes lower as the evaporation con- 

 tinues; and when the mercury arrives at a fixed point the diiference be- 

 tween the heights of the two thermometers shows the comparative state 

 of dryness of the air. I see there is a difference of seven degrees between 

 these two thermometers in this room. If the air be moist less evaporation 

 will take place, and the decrease of the temperature of the wet bulb will 

 be slight; but if the air is very dry tlie evaporation will be very great, and 

 the temperature diminished also. 



Self-Registering Measuring Instruments. 



The Chairman. ^The modern science of meteorology requires for its fur- 

 ther advancement frequent and regular observations of the changes in the 

 pressure, temperature and humidity of the air, and in the direction and 

 velocity of its currents, as well as of magnetic variations, as shown by the 

 dip and inclination of the needle. This service is performed by thousands 

 of observers in difierent locations over the whole surface of the globe. In 

 this country several hundred are under the direction of the Smithsonian 

 Institute. Their observations are made at three or four stated periods 



