Daniell on a new Hygrometer. 'i()l 



tube /. is closed hermetically by the flame of a la,mp. Thig 

 process is well known to those who are accustomed to blow glass, 

 and may be known to have succeeded, after the tube has become 

 cool, by reversing the instrument andtakingone of the balls in the 

 hand, the heat of which will drive all the ether into the other ball, 

 and cause it to boil rapidly. The other ball a. is now to be covered 

 with a piece of muslin. The stand g. h. is of brass, and the trans- 

 verse socket i. is made to hold the glass-tube, m the manner of 

 a spring, allowing it to turn and be taken out with little diffi- 

 culty. A small thermometer k. I. is inserted into the pillar of 

 the stand. 



The manner of using the instrument is this : After having 

 driven all the ether into the ball b. by the heat of the hand, it is 

 to be placed in an open window, or out of doors, with the ball b. 

 so situated as that the surface of the liquid 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 vapour in the interior of the in- 

 strument. The consequent evaporation from the included ether 

 produces cold in the ball 6., the degree of which is measured 

 by the thermometer, d. e. This action is almost instantaneous. 

 The thermometer begins to fall in two seconds after the ether 

 has been dropped. A depression of 30 degrees is easily pro- 

 duced, and I have seen the ether boil, and the thermometer 

 driven down below 0° of Fahrenheit's scale. The artificial cold 

 thus produced causes a condensation of the atmospheric vapour 

 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 instrument, such as a house or 

 a tree, as the cloud is not so soon perceived against an open 

 horizon. The depression of temperature is first produced at 

 the surface of the liquid where evaporation takes place, and 

 the currents which immediately ensue to restore the equi- 

 Vo).. VIII. X 



