58 REPORT— 1841. 



Of the Hygrometer. — It is desirable that two hygrometers should be pro- 

 vided, which may be fixed side by side upon the lid of a box, into which 

 they may be contrived to pack. The observer should not only note the tem- 

 perature of the first appearance of dew, but the temperature at which it 

 again disappears ; and wlaile he is waiting for the last observation by one in- 

 strument, he may proceed to make a new one with the other. A store of the 

 best aether should be provided, and a convenient dropping-bottle. No dis- 

 advantage would arise from the effect of the diminished pressure upon the boil- 

 ing-point of the aether, if placed in a bottle contrived for the purpose ; as thus, 



-r 



n, ab the bottle, 



3I c d the level of the asther, 



ef a tube fitted tight into the neck, and passing to the bottom of 



** the liquid, furnished with a stop-cock e. As the atmospheric 



^ pressure diminished upon the aperture of the stop-cock, the 



^ * pressure of the included vapour would pour out a stream of 



aether, which might be regulated, and the rapidity of its subsequent evapora- 

 tion would be a great advantage ; but as it is probable that the dryness of 

 some of the upper sections of the atmosphere may be extreme, smaller tubes, 

 filled with condensed sulphurous acid, should be provided, and kept cool in 

 ice, by the dropping of which upon the bulb of the hygrometer extreme cold 

 may be produced. As an additional precaution, a small bright silver capsule 

 and delicate spirit-thermometer may be prepared, by which the dew-point 

 may be observed from the direct evaporation of the acid. Bottles containing 

 a mixture of liquid carbonic acid and aether might perhaps be prepared, 

 which would answer the purpose still more perfectly. As it is extremely 

 desirable that the relation of the cold produced by evaporation from the sur- 

 face of a wet-bulbed thermometer with the dew-point should be ascertained, 

 and as such an observation would not add much to the trouble of the 

 observer. Dr. Mason's hygrometer, which is a convenient form of the instru- 

 ment, may be fixed upon a stem upon the box, immediately behind the 

 hygrometers, and the temperatures of the two thermometers may be noted. 

 The freezing of the water in the upper regions will, however, put an end to 

 these observations. The stem which supports the thermometers may also be 

 made to carry a moveable card-board, covered on the outside with gilt paper, 

 so as to screen all the instruments from direct radiant heat. 



Of the Barometer. — The only barometer that can be used, and can be 

 trusted in observations like those in question, appears to be the Siphon-baro- 

 meter of Bunten, in Paris (Quay Pelletier, No. 26), or barometers of a 

 similar construction by Robinson, of London. The tubes of Bunten appear 

 to be carefully made ; the column of mercury is easily seen ; and the slow 

 motion of the verniers, though not so fine as in Robinson's, is more easily 

 managed, a circumstance of some importance in the present instance. The 

 barometers should be new : their scales divided in millimetres only. Some 

 of them have a scale of English inches, which, owing to some mistake about 

 standard temperature, is very erroneous. They should be always kept in- 

 verted, except when in actual use. When allowed to hang in the position in 

 which they are used, the mercury in the short tube becomes oxidized, the 

 glass covered with a powder of the oxide, and the capillary depression con- 

 siderably increased, which renders the instrument useless. In a cistern- 

 barometer, where the level of the mercury cannot be observed, the cor- 

 rections for a change of level for small variations of barometrical pressure 

 are extremely troublesome. For large changes of barometrical pressure they 

 must become uncertain in the highest degree. Troughton's mode of deter- 

 mining the lower level is decidedly bad. The cistern-barometers, in which 



