552 



On a Portable Hygrometer, 



cessary to nearly fill it when the bulb occupies its axis ; this quanti- 

 ty once adjusted, serves for many subsequent experiments ; the tube 

 is then screwed to the cap, and the instrument being held in an in- 

 clined position, the thread on the inferior part of the tube is wetted 

 with ether ; in a few moments dew is deposited on the platina ferule, — 

 the instrument is then agitated, to ensure an uniform distribution of 

 heat within the tube ; the dew begins to disappear, — the temperature 

 then indicated by the thermometer is carefully noted. In the section- 

 al representation of the instrument, A is the brass tube with its cotton 

 envelope, B C the platina or steel ferule, D disk of caoutchouc and 

 screw cap, E the thermometer. 



Another form of the instrument, more elegant and costly, and fitted 

 only for determining the temperature of vapor of low tension, has 

 been used. S No. 2, represents a black glass tube, of the same capaci- 

 ty compared with the bulb as the brass tube ; it is attached by fusion to 

 the stem at R, and after being filled in part with alcohol, is hermeti- 

 cally sealed at O. The upper part of the black tube is the deposit- 

 ing surface, the lower being covered with thread ; the same mode of 

 experimenting as in the former case. 



No. 1. 



No. 2. 



For making corrections in barometrical 

 experiments, this instrument is peculiar- 

 ly fitted, the " Detached Thermometer" 

 serving to determine the temperature of 

 the air and tension of atmospheric vapor ; 

 the experimental result is substituted forjy 

 that deduced by calculation from the gen- 

 eral formula of LaPlace. Some results 

 recently obtained by my friend. Dr. Ed. 

 E. Phelps, show the great importance of 

 making the experimental, rather than the 

 calculated correction. 



The small size of the instrument, ena- 

 bles us to use a thermometer with large 

 divisions, the dew point can be found to a fraction of a degree ; the 

 quantity of matter to be refrigerated being small, a reduction of tem- 

 perature is speedily effected, and only an inconsiderable quantity of 

 ether is required. 



Roxbury Laboratory, SOtli Nov. 1829. 



