238 In tell (g en ce an d Miscellan eous Articles. 



perpendicularly, a thermometer with a blackened tube is introduced, 

 on which strike the solar rays directed on the axis of the cylinder. 

 A thick glass closes the posterior opening of the cylindrical tube, the 

 other remaining free ; but with a diaphragm of suitable aperture the 

 entire apparatus is protected from solar radiation. The entire appa- 

 ratus is mounted on a parallactic foot provided with very regular 

 movements. For the other details, and for the mode of using this 

 instrument, the Bulletin de V Observatoire du College Romain, vol. xi. 

 p. 14, may be referred to. I may simply mention that the thermo- 

 meter-scales are divided into fifths of a degree. 



By means of this arrangement there are eliminated (1) the agitation 

 of the air, and (2) the radiation of surrounding objects, the influence 

 and force of which may always be known, and varied at pleasure by 

 changing the temperature of the water of the envelope. There only 

 remains the radiation of the sun, and that of the portion of the hea- 

 vens visible from the place of the thermometer, and which, can be 

 diminished at will by placing in front of the instrument a diaphragm 

 scarcely larger than the bulb of the thermometer. As regards the 

 influence of the absolute temperature of the envelope, it is well 

 known by Mr. Waterston's observations that it has no influence on 

 the increase of temperature which solar radiation produces on the 

 blackened thermometer ; and I have assured myself, by varying the 

 temperature of the water between 7 and 70 degrees, that when the 

 temperature of the two thermometers is stationary, their difference 

 remains constant whatever their absolute value ; so that this differ- 

 ence (at least within certain limits) does not depend on the tempera- 

 ture of the envelope. This at first sight appears very singular ; for 

 it is curious to see that if the black thermometer rises to 19°' 6 when 

 the wr.ter is at 7°, when the temperature of the latter is at 70° the 

 black thermometer rises to 82° in the sun, the difference always 

 remaining 12 degrees. The practical difficulty of verifying these 

 facts depends on the rapidity of variation which the thermometer 

 placed in water undergoes for high temperatures ; but there is an 

 indirect verification in the fact that, after a certain limit, both ther- 

 mometers rise together, their difference always remaining con- 

 stant. 



With this apparatus I made a great number of observations during 

 summer, and I have repeated them in the present season in the per- 

 fectly bright days from the 22nd of November to the 8th of Decem- 

 ber, exposing the apparatus to the solar radiation under the dome of 

 the observatory until the relative temperature was constant for a 

 considerable time. The following are the principal conclusions at 

 which I have arrived. 



1 . During summer, observing near the meridian and near the sol- 

 stice, the relative temperature has varied from 14 to 11 degrees ; the 

 mean of several observations gives 12°*06. 



2. Continuous observations during the month of August give 

 values from 13 to 11 degrees. Their mean is still 12 degrees. 



3. Those of November and December give 12°'5 and 11°*5, and 

 the mean has not appreciably changed. 



