120 ANNUAL OF SCIENTIFIC DISCOVERY. 



in deep sea cables, with advantage, as it was nearly equal in conducting 

 power, and was only one-third of the weight. The outer covering should be 

 of hard material, so as to resist the longitudinal strain during the process of 

 submerging; but it should add as little as possible to the weight. It was 

 considered that no material fulfilled these conditions as well as soft steel. 



It had been found that the light and heat of the sun, the mycellium of a fun- 

 gus, and other substances and conditions, had the power of rendering gutta- 

 percha unfit for the insulation required for the transmission of messages by 

 means of electricity. Several specimens of gutta-percha, in a decayed state, 

 were exhibited; and also a piece of copper wire, five feet in length, covered 

 with gutta-percha, which was strained until it broke ; when the gutta-percha, 

 owing to its partial elasticity, contracted, and left seven inches of copper wire 

 uncovei-ed. A newly-made tube of gutta-percha, under a strain of 276 Ibs., 

 stretched from 14 inches to 24 inches before breaking; but a similar tube, 

 which had been exposed for about five years to the atmosphere, light, and 

 heat of the sun, was so brittle as to be easily broken by the hand. 



The London Builder publishes the following curious item of information 

 bearing on the subject of the duration of submarine telegraph cables : 



" On examining a piece of submarine cable cut from the end of the La 

 Manche line, long in use, there were noticed an indefinite series of ruptures 

 or subdivisions, as if the wire had been chopped into morsels, or had been 

 disintegrated, under the influence of the electrical vibrations. Since, in the 

 case of the transatlantic cable, currents positive and negative alternately are 

 launched through it, such a disintegration of the wire must be expected to 

 come about even more rapidly. The fact itself is too mysterious to be dis- 

 cussed at present." 



ON THE PRESENT STATE OF OUR KNOWLEDGE RESPECTING TER- 

 RESTRIAL MAGNETISM. 



From the report of a joint committee appointed on the part of the Royal 

 Society and the British Association to consider the expediency of memorial- 

 izing Government to reneAv the system of magnetic observations formerly 

 carried on at various colonial and foreign stations, but now suspended, we 

 derive the following information respecting the results thus far obtained 

 from the accumulated observations made at the several observatories at St. 

 Helena, Toronto, Hobarton, and the Cape of Good Hope. In the first place, 

 the committee report, that the mean state of the several elements for each 

 of the stations, as reduced to a fixed epoch, has been obtained with a precis- 

 ion of which nothing previously done has afforded any example emulat- 

 ing, in this respect, the exactness of astronomical determinations, and com- 

 petent to serve as a fixed point of departure, to the latest ages ; and this for 

 each of the elements in question the dip, the declination, and the intensity 

 of the magnetic force. 



Secondly, that at each station the rate of regular progressive secular 

 change, in all three of the elements above mentioned, has been ascertained, 

 with a degree of precision which contrasts strongly with the loose and inac- 

 curate determinations of former times. 



Thirdly, that the laws of the diurnal, annual, and other periodic fluctua- 

 tions in the value of these elements, as exhibited at each station, have been 

 established in a manner and with a decision to which nothing hitherto exe- 

 cuted in any branch of science, astronomy excepted, is comparable; and 

 that the results embodied in the examination of these laws have laid open 



