436 M. Oersted on [Juhe, 



If the last physiological inference is correct, it is highly inte- 

 resting in a pathological point of view. It enables us to explain 

 how hydrops thoracis, or that species of it called hydrops 

 pleurae, may exist to a certain extent, without being attended 

 with any symptoms indicating the presence of the disease, as 

 related by numerous medical authors. We can also more satis- 

 factorily account how the lung so frequently escapes being 

 wounded when weapons penetrate the cavity of the thorax; and 

 how the extravasation which follows, if not considerable, pro- 

 duces but little derangement. It may also have a practical 

 utility, for it informs the surgeon that the lung descends to a 

 certain point only, so that he need not be afraid wounding it 

 should an operation be required below that position. 



Dr. Carson retained open the apertures into the chests of his 

 animals with his fingers ; whereas mine were kept open in the 

 manner described, which accounts for the different results of our 

 experiments. Dr. Carson, in his Essays, does not allude to (his 

 circumstance, but since my investigations he has mentioned it. 

 When air is admitted into the cavities of the chest, the animal 

 requires the aid of all his respiratory powers. 



Article VII. 



OnM. Schweigger's Electromagnetic Multiplier, ivithan Account 

 of some Experiments made with it. By Prof. Oersted. (Com- 

 municated by the Author.) 



Immediately after the discovery of electromagnetism, 

 Prof. Schweigger, of Halle, invented an extremely useful instru- 

 ment for the purpose of discovering very weak electrical currents 

 by means of the magnetic needle. The effect of this multiplier is 

 founded upon the equal action which every part of a conducting 

 wire when it transmits a current exerts upon the magnetic needle. 

 When a part of this wire is curved as in A B C, fig. 1 (PI. XX), 

 so that the two branches A B and B C are in a vertical plane, 

 and a magnetic needle D E is properly suspended in the same 

 plane, it will be readily conceived that the needle receives an 

 impulse double that which it would receive from one only of 

 the branches. The impulse given by each branch has also the 

 same direction, since it is in fact the same side of the wire 

 which in both branches is opposite the needle. The effect is 

 still further increased when the conducting wire makes several 

 circumvolutions round the needle, as in fig. 2, and thus an elec- 



