94 THE SCIENTIFIC PAPERS OF 



of it, a Weber's reflecting magnetometer. These are arranged 

 together in the manner of a Wheatstone's bridge or balance, the 

 cable to be measured forming the fourth resistance, or the unknown 

 quantity in the equation, expressing the condition of a balance 

 between the adjusted resistances. By an instrument of this con- 

 struction electric resistances varying from the one hundredth 

 part of a unit, to one million units, can be measured with great 

 accuracy. 



For resistances exceeding one million units, a different method 

 was adopted, in which the resistance was calculated from the de- 

 flection of a very delicate sine galvanometer, acted upon by a 

 battery of ascertained electro-motive force. The limits of this 

 paper will not admit of a detailed description of the testing appa- 

 ratus which was used, or of the mathematical formulae which were 

 employed in reducing the observations into comparative numerical 

 measurements.* 



The diagrams in Plates 6 and 7 represent graphically the 

 results of observations upon two different sections of the cable, 

 taken at various stages of their progress : the one section being 

 between Malta and Tripoli, and the other between Alexandria and 

 Benghazi.f In Plate 7 are shown the tests applied to the 

 parts constituting the Malta and Tripoli section of the cable, 

 with the exception of a few knots, the presence of which has no 

 perceptible influence upon the condition of the whole. In the 

 diagrams marked A A, the insulation tests at the Gutta Percha 

 Works are given. The length of the coils are taken as abscissas, 

 and the ordiuates as resistances at 75 Fahr. The black line con- 

 nects those taken in vacuo, and the broken line those taken under 

 a pressure of 600 Ibs. per square inch. The lower horizontal 

 broken line represents the standard of 90 millions of units at 

 75 Fahr., to which it was found necessary to reduce the original 

 standard of 100 millions, in consequence of the inability of the 

 Gutta Percha Company to provide sufficient material of such high 

 insulating qualities. The upper horizontal black line and the 



* For further details on these subjects, see "Report of the Joint Committee on 

 the Construction of Submarine Telegraph Cables," Appendices Nos. 7 and 12. 

 C. W. S. 



) These observations are given in a series of Tables which are appended to the 

 original communication, and may be consulted at the Institution. 



