of a New Hygrometer,. 33 



rainy season ; from which we might infer, that when the 

 temperature of the dry bulb remains the same, the moistened 

 one rising one or two degrees, rain may be expected, or 

 the contrary. Supposing the moistened bulb sinks 2 de- 

 grees more than ordinary, during the same day, it would in- 

 dicate a continuance of fair weather. 



Third objection. That the two thermometers are more 

 or less affected by reflected light. 



By covering the two bulbs with the same substance any 

 relative difference will be prevented, as both bulbs will be 

 equally affected by radiant and reflected light and heat. 



It appears from many experiments I have made on the 

 subject, that radiant heat from the sun, does not influence 

 the process of refrigeration, as the temperature of both bulbs 

 rises equally from that cause. 



But in stating my opinion, that the hygrometer now pro- 

 posed is not affected by radiant heat from the sun, I wish it 

 to be distinctly understood, that this is only the case when 

 the instrument is suspended in free space, at some distance 

 from the ground, and apart from bodies giving off radiant 

 caloric ; for when the hygrometer is subjected to the influ- 

 ence of radiant caloric given off from heated bodies, its indi- 

 cations are no more to be depended upon as furnishing ac- 

 curate results than those afforded either by Sir John Leslie's 

 or the dew-point hygrometers ; the indications afforded by 

 my hygrometer being erroneous in proportion to its proxi- 

 mity to the source of error. 



However, I can with confidence assert, that the instru- 

 ment may be used under circumstances in which it would 

 be impossible to obtain correct data, either with Sir John 

 Leslie's or the dew-point hygrometers. 



All those instruments should unquestionably be used as 

 much as possible in the shade, but from repeated and varied 

 experiments, I am convinced, that the hygrometer now 

 proposed, will be less affected by this source of error, than 

 any other at present in the hands of the meteorologist. 



Fourth objection. That evaporation is influenced by the 

 density of the atmosphere, being greater or less, according 

 to the height of the mercury in the barometer, consequently, 

 that corrections would be required to rectify this error. 



This objection I consider without foundation, being in- 



VOL. IV. D 



