110 Prof. Wartmann on the Polarization of Atmospheric Heat. 



The body of the pile is sheltered against variations of tempe- 

 rature by filling the entire chest with carded cotton. In the side 

 opposed to that which contains the analyser is a rectangular 

 glazed window, through which by means of a good thermometer 

 the temperature of the envelope can be read off. Finally, a little 

 hole pierced in the bottom permits of the passage of two wires 

 from the poles of the pile to the rheomcter. The whole is pre- 

 served in a place less warm than the surrounding atmosphere, 

 so that during the experiments the pile must necessarily be 

 affected by any accession of heat. The sense of the deviation of 

 the rhcometer serves to prove that this condition is fulfilled. 



The chest furnished with horizontal axes of hard wood turns 

 in a rectangular frame, which permits of the pde being retained 

 at any angle whatever with the horizon, in the vertical plane 

 which it describes. This angle of declination is estimated on an 

 appropriate dial by means of a plummet and an index which fol- 

 lows the chest in its motion. The frame, in its turn, moves 

 round a vertical foot, in which it is steadied by friction. The 

 azimuths are read on a fixed horizontal dial, which permits of 

 the adjustment of the apparatus. No magnetic metal ought to 

 be used in the construction of the latter. 



I have said that the temperature of the air is subjected to 

 almost perpetual fluctuations, which cause corresponding varia- 

 tions in the thermo-electric current. To lessen this grave in- 

 convenience, the pile was caused to act near the window of a 

 closed room. In the reading of the rheometric deviations, it is 

 better to determine the arcs described by the index at each 

 change of the plane of analysis, than the positions at which it 

 tends to come to rest after a number of excursions, which become 

 less rapid the more nearly astatic is the system of needles. The 

 results agree exactly with those deduced from fixed deflections, 

 in those rare cases when the atmosphere is calm and permits of 

 the operation being carried on in the open air, as also within 

 doors. 



The success of these researches depends also upon the good- 

 ness of the rheometer. I have obtained an excellent multiplier 

 from M. Rhumkorff. It is composed of two short and thick 

 wires rolled on a frame of bone. The dial is of pure copper, 

 with its graduated circumference silvered. The needles, sus- 

 pended from a fibre of silk extremely fine, and 015 of a metre 



placed at a convenient distance, traversing the second Nicol's prism which 

 serves as analyser. Although it has to pierce a total thickness of 0'185 of 

 a metre of diathermic bodies, it retains the power to produce a current very 

 appreciable by the rheometer. The difference of the value of deviation, 

 according as the magnet is or is not in a state of activity, entirely confirms 

 the results which I obtained three years ago. 



