METEOROLOGICAL INSTRUMENTS. 133 



may be upon a level with the eye. A few drops of ether 

 are then to be poured upon the covered ball. Evaporation 

 immediately takes place, which, producing cold upon the 

 ball a, causes a rapid and continuous condensation of the 

 ethereal vapor in the interior of the instrument. The con- 

 sequent evaporation from the included ether produces cold 

 in the ball b, the degree of which is measured by the ther- 

 mometer, d e. This action is almost instantaneous. The 

 thermometer begins to foil in two seconds after the ether has 

 been dropped. A depression of thirty degrees is easily 

 produced, and sometimes the ether boils, and the thermo- 

 meter falls below of Fahrenheit's scale. The artificial 

 cold thus produced causes a condensation of the atmospheric 

 vapor upon the ball, b, which first makes its appearance in 

 a thin ring of dew, coincident with the surface of the ether. 

 The degree at which this takes place is to be carefully noted. 

 A little practice may be necessary to seize the exact moment 

 of the first deposition, but certainty is very soon acquired. 

 It is advisable to have some dark object behind the instru- 

 ment, such as a house, or a tree, as the cloud is not so soon 

 perceived against an open horizon. The depression of tem- 

 perature is first produced at the surface of the liquid where 

 evaporation takes place, and the currents which immediately 

 ensue to restore the equilibrium are very perceptible. The 

 bulb of the thermometer, d e, is not quite immersed in the 

 ether, that the line of greatest cold may pass through it. 

 The greatest difference that Mr. Daniell has observed in the 

 course of four months' daily experiments between the ex- 

 ternal thermometer, k I, and the internal one, e d, at the 

 moment of precipitation in the natural state of the atmo- 

 sphere, was twenty degrees. In very damp weather the 

 ether should be slowly dropped upon the ball, otherwise 

 the descent of the thermometer is so rapid as to render it 

 impossible to be certain of the degree. In dry weather, on 

 the contrary, the ball requires to be well wetted more than 

 once, to produce the requisite degree of cold. It is almost 

 superfluous to observe, that care should be taken not to 

 permit the breath to affect the glass. With these pre- 

 cautions the observation is simple, easy, and certain. 



By combining the rise and fall of the barometer with the 

 effects of this instrument, we learn to modify their results, 

 and by so doing can hardly be deceived in the weather for 

 many hours in advance. The indications are to be corrected 



VOL. II. 12 



