332 Dr. Seebeck on the unequal Evolutio7i 



the same side could not for the most part be noted. The ef- 

 fect might, indeed, be raised by means of lenses ; but I seldom 

 made use of them, considering that the more an apparatus is 

 complicated, the more difficult it is to arrive at a certain re- 

 sult by its means, and that they could only be properly a}ipli- 

 ed with a heliostat, in which all the parts of the appai-atus can 

 be securely fixed, but which I did not possess. Air-thermo- 

 meters seemed upon the whole to deserve the preference, since 

 their great sensibility led me to expect that they would give 

 decided indications even immediately in the prismatic light, 

 and for small differences of temperature. I first intended to 

 employ one of Leslie's photometers ; yet as the one I ordered 

 was not delivered at the right time, I determined to try whether, 

 and to what extent, an air-thermometer with a simple bulb 

 might be used in these experiments. The result surpassed 

 my expectation, and I therefore preserved it in all my subse- 

 quent trials. There were certainly some difficulties in the use 

 of this instrument, but they were diminished by more practice. 

 One difficulty, howevei", I could not obviate ; viz. the varia- 

 tions in the thermometer, produced by the formation of clouds, 

 or even any slight vapours rising before the sun (a thing which 

 frequently occurs even when the atHiosphere is apparently 

 clear). Thei'e was no other remedy against it except frequent 

 repetition of the experiments with the same prisms in the 

 clearest days, — those most free from vapours; and this I em- 

 ployed accordingly. The best days for such experiments were 

 those after a thunder-storm, or in clear weather after rain; 

 when the results accorded best. From a great number of 

 exjjeriments, I shall here give only those which are marked 

 in my journal as having been successful, — such as in which 

 neither the height of the barometer, nor of the mercurial ther- 

 mometer in the place where the experiments were being made, 

 underwent any material alteration, at least during the time of 

 one particular experiment. 



The apparatus just mentioned, consisted of a thermometer 

 tube 15 inches long, with a very thin bulb of half a Paris 

 inch in diameter. The bulb was uniformly stained with China 

 ink ; and a scale made of thin pasteboard, diviiled into Paris 

 inches and lines, was fixed against the tube. This scale was 

 removed one inch from the bulb, and had there its zero. 

 Thus the bulb and one inch of the tube were quite free. Within 

 the tube was a drop of a coloured liquid, which, after the whole 

 length of the tube had been properly moistened, filled about 

 one inch. The instrument, thus prepared, was placed on a 

 stand which could be raised and lowered at pleasure, and 

 moved, sometimes in a horizontal sometimes in a vertical po- 

 sition, 



