520 



REPORT 1867. 



tion, and for horizontal magnetic intensity were alternated in such a manner 

 that each class occupied the same portions of the day that the others did. I 

 sought in this way to avoid the effects of any horary change in the humidity 

 &c. of the atmosphere, or in the magnetic force. Of the thirty experiments 

 comprising each class, six were performed at about each of the several hours, 

 11 A.M., 12| P.M., 1| P.M., 4 P.M., and 5| p.m. 



The calorimeter, protected as already described, was supported on the 

 edges of a light wooden frame. It was carefully guarded against draughts 

 by screens coated with tinfoil placed at a foot distance. The stirrer con- 

 sisted of a vertical copper rod, to which vanes, on the plan of a screw- 

 propeUer, were soldered at four equidistant places. The rod extended 

 2 inches above the calorimeter, and was there affixed to a light wooden shaft 

 2 feet long, attached at the upper end to the last spindle of a train of clock- 

 wheels. The weight was 35 lbs., which, falling about 2 feet per hour, pro- 

 duced a continuous revolution of the stirrer at a rate of about 200 in the 

 minute. The action of the stirrer left nothing to be desired. It was started 

 five minutes before an experiment commenced, and kept going until the last 

 observation of the thermometer had been made. 



The experiments, as in the second series, lasted one hour, during which 

 were made eight observations of the thermometer immersed in the calori- 

 meter, twenty of the temperature of the air, and forty of the deflection of the 

 galvanometer. 



