14 REPORT ISG^. 



per unit of length, in electrostatic absolute measure ; c tlie capacity per unit of 

 length in the same measure ; and I the total length of the wire ; k is known for all 

 the ordinary metals, but c has hitherto been undetermined ; and the object of the 

 paper was to deduce the value of c, from some experiments made by JI. Guillemin, 

 and fully detailed in the ' Annales de Physique et de Chiniie ' for 1860. These 

 experiments gave with considerable accuracy the form of the cuiTe for various 

 lengths ; but the experimenter had not applied his results so as to give the con- 

 stants required for the mathematical theory. After describing the method employed 

 by M. Guillemin with high commendation, Mr. Jeukin gave the results of his cal- 

 culations. The electrostatical capacity per foot of the common No. 8 wire in the 

 lines used by M. Guillemin must have been from 0-15 to 0-22 in British absolute 

 electrostatic measure (feet, gi-ains, seconds). This number is nearly three times 

 that given by pure theory for a wire, stretched horizontally, without supports, at 

 a uniform b eight of ten feet from the ground — a discrepancy probably to be ac- 

 counted for by the induction occurring at each post. The form of the curve was 

 also modified" by imperfect insulation. The retardation due to the statical charge — 

 the capacity for which is thus determined — not oidy delays the signals, but causes 

 confusion and utter illegibility if they succeed each other too rapidly. A limit is 

 thus put to the perfonnance of signalling-instruments ; and calculations made with 

 the above value of c show that we must not expect to transmit by the common 

 Morse instrument more than about twenty words per minute between stations loOO 

 miles apart ; that the performance of Prof. Wheatstone's beautiful automatic ti-ans- 

 mitter may be limited to speeds below 120 words per minute when 530 miles fire 

 exceeded ;" and that the Chevalier Bonelli would have to diminish his speed of 400 

 words per minute '(with five wires), even on considerably shorter circmts. It must 

 be remembered that larger wires, fewer posts, and a better form of insulator may 

 considerably extend these limits. 



Description of an Electric-resistance Balance constructed by Prof. W. Thomson. 



By FiEBimfG Jenkin. 

 The author described an instrument made under the superintendence of Prof. W. 

 Thomson, of Glasgow, for the purpose of practically carrying out the iniportnnt 

 improvements in the methods of comparing the electric resistances of short thick 

 bars of wires of metal, as described in a paper by Prof. Thomson, published in the 

 ' Philosophical Magazine.' The special merits of the method are, that the bars or 

 wires to be tested do not require to be cut to any definite length, that they do not 

 require to be soldered or joined by amalgamated terminals to the connexions of 

 the instrument, and that any resistance due to slightly imperfect connexion between 

 the bar tested and these connexions does not vitiate the measurements. This im- 

 portant practical improvement, by which the accuracy of the measurement and the 

 ease with which it can be effected are alike increased, was explained by reference 

 to the fine instrument exhibited, which has been constructed for Col. Douglas, 

 Superintendent of the Telegraphs in India, by Mr. James White, of Glasgow. 



On the Develojnnent of Electricity from the Hays of the Stm and other Sources 



of Light. By H, Keetil. 



Descriptions of tJie " Liquid Steering Compass " and " Monitor Compass." 

 By Professor H. D. PiOGees. 



The compasses described were constructed by Mr. E. S. Ritchie, of P.oston,U. S. 

 The distinctive peculiarities of the liquid compass are an air-tight metallic case, 

 within which is placed the magnetic needle, and of such size and weight as to be 

 of very nearly tlie same specific gravity as the liquid in which it is intended to 

 float. The weight is thus removed from the pivot, and fi-iction is almost pre- 

 vented; certain modifications being introduced to provide against tilting and other 

 emergencies occurring during the motion of the ship. Tbe distinctive principle of 

 the monitor compass is the separation of the magnet from the card or index, so 

 that tlie magnet may be elevated above the sphere of disturbing attraction of the 



