OP ARTS AND SCIENCES. 209 



of the calorimeter and the height of the barometer were accurately 

 observed, the first to the hundredth of a centigrade degree and the 

 hist to the twentieth of a millimeter, noting always the height of the 

 attached thermometer. The hand was then quickly thrust into the 

 calorimeter, and the outlet cock at once closed. The balloon was now 

 removed, dusted with a large camel's-hair brush, and hung in its place 

 on the bahmce. When, however, the atmosphere is very dry, as it 

 often is in this climate during the winter months, it may happen that 

 the surface of the glass acquires a charge of electricity from the fric- 

 tion of the cushion in the calorimeter or of the brush just mentioned ; 

 and if there is any suspicion of such an effect, the best way to dissipate 

 the charge is to stand the balloon for a few minutes in its metal case 

 (shown in Plate III.), after covering the bottom of the case with a thin 

 laver of water. If the balloon is hun*: on the balance with a charire of 

 electricity on its surface, however small, it will be unequally attracted 

 by the surrounding metallic walls, and most confusing anomalies ot 

 weight will be noticed. Moreover, in the very dry atmosphere of the 

 case the charge will last for days, or even weeks, as on the insulated 

 vanes of an electrometer. 



Thermometers and Barometers, and their Correction. 



The thermometers on whose indications we have relied in this 

 investigation are two of small range, but divided into fiftieths of a 

 centigrade degree and made by the house of the late Dr. H. Geissler 

 of Bonn. We were obliged to use the two, for our investigation was 

 continued into summer weather, which exceeded the range of the ther- 

 mometer first selected. As should always be the case with such in- 

 struments, the zero point is given in each case on a short subsidiary 

 scale, separated from the main scale by an enlargement of the tube. 

 Careful observation showed that the zero point of the instrument we 

 will designate as No. 1, was depressed 0.02 of a centigrade degree, 

 while the zero point of No. 2 was raised 0.28 of a degree. IMaking 

 the correction thus indicated, to be added to all temperatures observed 

 with No. 1, and subtracted from all observed with No. 2, the two in- 

 struments were found to agree exactly through all poitions of the 

 scale common to both. Obviously, therefore, the differences of tem- 

 perature observed are perfectly trustworthy to the one hundredth of a 

 degree, but how far the absolute values — counting from the freezing 

 point of water — can be relied on when compared with the observa- 

 tions of other experimenters, we have no means of determining. Of 

 course, it will be understood that all temperatures hereafter given 



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