54 Mr. Jones's description of an improved hygrometer. 



SL Stand for the instrument. The case contains a small bottle 

 for ether. 



The thermometer thus constructed, will give both the tem- 

 perature of the air and that of the dew point ; which last is 

 effected by placing the mouth of the bottle containing the 

 ether, in contact with the upper part of the covered surface 

 of the bulb, when by gently inclining the bottle, the ether 

 will flow downwards without wetting the top of the bulb, 

 which will almost immediately become dull by the deposition 

 of moisture on its surface ; when the observed temperature 

 may be taken and the difference ascertained. 



Should it be objected against the principle of the instru- 

 ment here proposed, that the indications do not exhibit the 

 true temperature of the upper surface of the bulb on which 

 the deposition of dew takes place, but that of the lower part 

 to which the ether is applied ; it may be answered, that by 

 inclining the whole instrument so as to render the axis of the 

 bulb horizontal, and establish thereby a free circulation of 

 the mercury in every part, this objection may be obviated ; 

 but on repeated trials I have not found this to produce any 

 difference in the results. 



I ought also perhaps to mention that an instrument some- 

 what similar in principle has been used in Vienna, and was 

 mentioned by Professor Baumgarten of that capital to a 

 friend, who communicated the fact to myself. 

 The instrument is represented in Plate I. Fig. i . 



THOMAS JONES. 



20th February, 1825. 





