22 REPORT — 1855. 



klava wire, if of the same lateral dimensions, would give thirty-six times the 

 retardation, and thirty-six times the slowness of action. If the distinctness of 

 utterance and rapidity of action practicable with the Varna and Balaklava wire are 

 only such as to be not inconvenient, it would be necessary to have a wire of six 

 times the diameter ; or better, thirty-six wires of the same dimensions ; or a larger 

 number of still smaller wires twisted together, under a gutta percha covering, to give 

 tolerably convenient action by a submarine cable of six times the length. The theory 

 shows how, from careful obsers'ations on such a wire as that between Varna and 

 Balaklava, an exact estimate of the lateral dimensions required for greater distances, 

 or sufficient for smaller distances, may be made. Immense economy may be prac- 

 tised in attending to these indications of theory in all submarine cables constructed 

 in future for short distances ; and the non-failure of great undertakings can done be 

 ensured by using them in a preliminary estimate. 



On new Instruments for Measuring Electrical Potentials and Capacities. 

 By Professor W. Thomson, M.A., F.R.S. 



In this communication three instruments were described and exhibited to the 

 Section : the first a standard electrometer, designed to measure, by a process of 

 weighing the mutual attraction of two conducting discs, the difference of electrical 

 potential between two bodies with which they are connected, an instrument which 

 •will be useful for determining the electromotive force of a galvanic battery in electro- 

 static measure, and for graduating electroscopic instruments so as to convert their 

 scale indications into absolute measure ; the second aft electroscopic electrometer, 

 •which may be used for indicating electrical potentials in absolute measure, in ordinary 

 experiments, and, probably with great advantage, in observations of atmospheric elec- 

 tricity; and the third, for which a scientific friend has suggested the nameof Electro- 

 platymeter, an instrument which may be applied either to measure the capacities of 

 conducting surfaces for holding charges of electricity, or to determine the electric 

 inductive capacities of insulating media. 



On the Means proposed by the Liverpool Compass Committee for carrying 

 out Investigations relative to the Laws which govern the deviation of the 

 Compass. By John T. Towson. 



Experimental Demonstration of the Polarity of Diamagnetic Bodies. 

 By Professor Tyndall, F.R.S. 

 The author referred to the Bakerian Lecture of the present year, as proving that a 

 bar of bismuth freely suspended within a spiral of copper wire, excited by a current 

 passing through that wire and acted upon by external magnets, could be attracted 

 and repelled with the same certainty as, though with a far less energ\' than, a bar of 

 iron, the sense of the deflection, -which indicated the polarity of the diamagnetic 

 bismuth bar, being always opposed to the deflection of the iron bar under the same 

 circumstances. The experiments now described formed the complement, so to speak, 

 of those described in the lecture referred to. In the latter case, the bismuth bar was 

 deflected by magnets ; but as the action is mutual, it is to be expected that the magnets, 

 if properly arranged, could be deflected by the diamagnetic bars. An experiment of this 

 nature has already been made by Prof. Weber of Gottingen, but the results obtained by 

 this distinguished experimenter have not commanded general conviction ; they have 

 been questioned by Matteucci, Von Feilitzsch, and others. Prof. Tyndall has to thank 

 M. Weber for the plan of an instrument, constructed by M. Leyser of Leipsic, which 

 has enabled him to remove the last trace of doubt from this important question. 

 The instrument consists essentially of two upright spirals of copper wire about 18 

 inches long, fastened to a stout slab of wood, enclosed on all sides during the time 

 of experiment, and so fixed into solid masonry that the spirals are vertical. 

 Above the spirals is a wooden wheel with a grooved circumference; below the spirals 

 there is a similar wheel ; an endless string passed tightly round both wheels, and 

 to this string are attached two cylinders of the diamagnetic body to be examined. 



