128 Dr. TyndalFs Remarks on the Researches of Dr. Goodman. 



described by Dr. Goodman, in eases where there was no possi- 

 bility of an electric current being formed. 



During the inquiry on diamagnetism and magnecrystallic 

 action, an account of which appears in the September Number of 

 this Magazine for 1851, the torsion balance there described was 

 placed before a window through which the sun shone dui'ing the 

 forenoon. In experimenting with the spheres of bismuth, I was 

 often perplexed and baffled by the contradictory results obtained 

 at different hours of the same day. With the spheres of calca- 

 reous spar, where the diamagnetic action was weak, the discre- 

 pancies were still more striking. Once while gazing puzzled at 

 the clear ball of spar resting on the torsion balance, my atten- 

 tion was attracted to the bright spot of sunlight formed by the 

 convergence of the beams which traversed the spar, and the 

 thought immediately occurred to me that this little " fire-place " 

 might be sufficient to create currents of air strong enough to 

 cause the anomalies observed. The light was shut out, and 

 thu3 the source of my perplexity was effectually cut away. 

 The air-currents, however, were far more owing to the warming 

 of the glass cover of the instrument than to the convergence of 

 the sun's rays ; during the whole inquiry I was obliged to ex- 

 periment every sunshiny forenoon with closed shutters. On 

 mentioning this fact to Prof. Magnus, he informed me that 

 during his investigation on thermo-electric currents, a report of 

 which appears in the present Number of the Philosophical Maga- 

 zine, he was obliged to protect his galvanometer from the action 

 of the sun, as the unequal heating of its glass shade rendered his 

 astatic needles quite unsteady. It is almost incredible how 

 slight a difference of temperature is sufficient to create these 

 currents, and thus disturb the action of a finely suspended needle. 

 M. Kohlrausch, whose refined experiments I had the pleasure of 

 witnessing for several successive days last spi'ing, has been obliged 

 to construct a table for the express purpose of making allowance 

 for the little whirlwinds which sometimes establish themselves in 

 his electrometer. In his instrument, a needle of silver wire is 

 suspended from a glass fibre of extreme tenuity, the needle being 

 protected by a vessel of brass with a glass cover. The days on 

 which we experimented were cold ones ; and as long as a good 

 fire was kept in the stove which heated the room, the experiments 

 were satisfactory ; but as soon as the fire became low, and radia- 

 tion set in strongly against the cold window-panes before which 

 the electrometer was placed, the action of the air within the 

 brass vessel upon the needle was at once exhibited, and increased 

 to such a degree, with the decreasing temperature, that further 

 experiment had to be abandoned. M. Kohlrausch has mapped 

 these little currents with great care. They resemble, to compare 



