219 



Art. XX. Miscellaneous Intelligence. 

 Mechanical Science. 



§ Optics, Agriculture, the Arts. 



1. Extract of a Letter from Professor Oersted, of Copenhagen. 



I can at present only give you the result of an investigation, 

 which is so far void of interest as it only relates to the confir- 

 mation of an earlier discovery, I mean that of Zamboni, concern- 

 ing the electricity produced by the contact of a single solid con- 

 ductor with a single fluid conductor. You know that such 

 experiments are extremely delicate ; they seem to have been 

 repeated only by Mr. Erman, and this celebrated and ingenious 

 philosopher complains much of the irregularities which the 

 experiments present. I have found in the electro-magnetic mul- 

 tiplier, invented by Schweigger, a mode of making these ex- 

 periments with the greatest ease. I take two plates of zinc, 

 of different breadths, one, for example, 3 lines broad, the other 

 Ij ; I place them in a diluted acid, and I make a communication 

 between each of them and one of the extremities of the metallic 

 wire of the multiplier. The action is thus rendered very sen- 

 sible. The wider plate assumes in this galvanic arc the place 

 of the copper, the narrower that of the zinc. When we take 

 two plates which are perfectly equal, and attach them to the 

 extremities of the multiplying wire, we obtain no effect, if we 

 plunge both plates at once into the liquid ; but if we immerse 

 one before the other, that which has come the last into contact 

 acts as a less oxydable metal. 



Heights of Barometer., 25th Dec. 1821. 



2. Remarkable Dichroism of Tourmaline. — A very interesting 

 specimen of dichroitic tourmaline, in the cabinet of Mr. Allan, 

 exhibits the most singular contrast of colours ever yet found in 

 any substance. Tiie plate is cut perpendicular to the axis of 

 double refraction, and also to the axis of the prism. In the 

 direction of the axis the colour is a deep and brilliant blue, 

 while in a direction at right angles to the axis the colour is a 

 very pale red, approaching to pink. — Edin. Phi!. Jour. vi. 177. 



