INTHUDL'CTION. 



X.\.\1X 



TAISI.I-: vii. 

 Equivalent*, Centigrade and Fahrenheit thermometric scales. 



The bulb of one of the thermometers was enveloped in thin muslin, which was kept con- 

 stantly moistened by the capillary action of a short piece of lampwick that dipped in the water 

 of a bird-glass fitted on the wooden upright. 



Maximum and minimum self -registers . The self-register for the maximum temperature is a 

 mercurial thermometer with a cylindrical bulb, three and a half inches long, and four-tenths of 

 an inch in diameter, the tube being bent so that the bulb lies parallel with it. Its scale is 

 divided on boxwood to 1 Fahrenheit, from to 140, and its register index is a short piece of 

 blue steel wire. The self-register for the minimum temperature is of slightly colored but trans- 

 parent alcohol, with a bulb of the same dimensions and position with respect to its tube as the 

 preceding. Its scale also is of similar material, and equally minute in its divisions, from 10 

 to -f 140 Fahrenheit. Its register is a small float of dark glass with a knob at each end. 

 The two instruments were made by Henry Barrow & Co., London, and may be secured together 

 by a long brass pin that passes through the frame of one and screws into the other, or they 

 may be suspended separately. They were used together, and the floats were adjusted after the 

 observations at midnight, 



Radiation thermometers. The thermometer for radiation to the sky is of alcohol, with a 

 transparent bulb placed in the focus of a plated and burnished parabolic mirror. Its divisions 

 are engraved on the tube to 1 Fahrenheit from to 150. Its register index is a delicate 

 cylinder of colored glass, with a small knob at each end. The mirror rests on a ball and socket- 

 joint that permits it to be directed to any portion of the sky, and when in use the instrument 

 was placed upon the ground, above which the base of the ball and socket-joint elevates it about 

 five inches. There was no suitable place for this instrument on our premises. Radiation from 

 the heated walls of the court-yards was probably unceasing during clear weather ; and as soon 

 as this became absolutely certain, observations were discontinued, and the thermometer being 



