436 



METEOROLOGICAL INSTRUMENTS. 



Fig. 5 represents a convenient form for mounting a thermometer 

 for determining the temperature of grass due to radiation. 



Fig. 8. Fig. 7. 



Fig. 6 represents one of a series of 

 standard thermometers, extra sensitive, 

 about 20 inches in length, each degree 

 three-fourths of an inch, divided into 

 tenths or twentieths. 



Sensitive thermometers for extremely- 

 low temperatures are also constructed of 

 the same pattern, thirty-five inches long, 

 with a range from 60° below to 80° above 

 zero, filled with pure alcohol of the spe- 

 cific gravityof 120. 



Fig. 7 represents Regnault's condens- 

 ing dew-point hygrometer. 



This instrument consists essentially of 

 two sensitive thermometers, as shown in 

 1 the figure, the lower exposed to the 

 jn| action of the atmosphere, the upper to 

 ■J [tithe influence of a current of air passing 

 ;£| through ether contained in a well-pol- 

 | ished silver bottle, from the mouth of 

 which the stem of the thermometer 

 projects. This thermometer marks the 

 exact temperature at which the aqueous 

 vapor at the time in the atmosphere is 

 condensed in the form of dew upon the 

 bottle, and thus gives by direct observa- 

 tion the existing "dew-point." The 

 polished silver bottle is about one inch 

 in diameter, the neck being contracted 

 to about five-eighths. The thermometer inserted into this 

 bottle is a sensitive one, divided on its stem to half degrees, 

 the stem passing through an ivory stopper fitted with a 

 cork which renders the bottle air-tight at the neck. On one side, and 

 within the silver bottle, a small, slender silver tube descends to nearly 

 the bottom; this tube passes outwards, and is connected with an India- 

 rubber tube. Upon nearly filling the large part of the silver bottle 

 with ether, and -blowing through this tube, the air rises through the 

 ether in bubbles and carries with it a portion of the ether in vapor. 

 This evaporation of the ether causes such a degree of cold that the 

 surface of the silver bottle is so reduced in temperature as to cause a 

 precipitation of dew. The supporting stem of the instrument being 

 hollow a ready means is provided for the egress of the air. The bottle 

 at the foot of the stand is for containing a supply of ether. 



Fig. 8 represents the hygrometrical apparatus of instrument for 

 measuring altitudes by the boiling point of water. It consists first, 

 of a strong sensitive enamelled thermometer, the scale of which ranges 

 from 180° to 214° Fahrenheit, divided on the stem so as to show the 

 tenth of a degree. Second, a copper boiler supported on a small tripod 

 and surmounted by a telescopic draw-tube, which is again sur- 



