Prof. Meyen on the Progress of Vegetable Physiology. 881 



adapted to measuring the deflecting forces of the galvanic bat- 

 tery. By using a thick wire it was ascertained that the calori- 

 motor of Professor Hare having 40 plates, each 18 inches 

 square, acted on the needle with a force equal to 92 grains, 

 applied at the distance of 6 inches from the centre. In at- 

 tempting to force the needle by torsion into a line parallel to 

 the coil, where the deflecting current acts with the greatest 

 strength, I accidentally carried it too far and reversed its 'po- 

 sition^ when instantly it became reversed m polarity^ that which 

 had been the north pole becoming the south. This showed 

 how unfit is the magnetic needle to measure such a quan- 

 tity of electricity as was then flowing through the massive 

 conductor. The instrument is well adapted to show to a class 

 the experiments upon radiant heat with Pictet's conjugate 

 reflectors, in which the differential or air thermometer affords, 

 to spectators at a distance, but an unsatisfactory indication. 

 For this purpose the electrical element necessary is merely 

 a disk of bismuth as large as a shilling, soldered to a corre- 

 sponding one of copper, blackened, and erected in the focus 

 of the reflector, while conductors pass from each disk to the 

 poles of the galvanometer. With this arrangement the heat 

 of a non-luminous ball at the distance of 12 feet will impel 

 the needle near 180°, and if the connexions and reversals are 

 properly made will keep it in a continued revolution. 



I have thus given you a brief sketch of an instrument which 

 seems to supply a desideratum on the lecture-table, when the 

 common thermometer is too small to afford to a class that 

 direct and full satisfaction which, in a subject so important as 

 that of heat, is very desirable to every professor. I have not 

 so far attempted to use it extensively as an instrument of re- 

 search, yet it shows evidently the importance of massiveness 

 in conductors for feeble currents, such as those produced by 

 thermo-combinations; nor am I certain that I have arrived 

 at a maximum in this particular, for so far as I have proceeded 

 in using thicker conductors for the coil the deflecting effects 

 have been increased. I am, &c. 



London, Aug. 30, 1837- JoHN LoCKE. 



XLVIII. A Report of the Progress of Vegetable Physiology 

 during the Year 1836. -By J. Meyen, Professor of Botany 

 in the University of Berlin.* 



IT is delightful to see that during the past year not only have 

 the works published in the province of botany been in- 



* From Wiegmanu's Arch'm fur Naturgeschichtc, 1837, Part 3. Trans- 

 lated by Mr. Win. Francis. 



