By our last foreign exchanges we observe that the skillful electricians having this matter in charge are seriously concerned as to whether, when the cable is successfully laid down, there will not still remain the most gigantic electrical experiment ever made. Will it be possible to send the electrical signal through more than two thousand miles of wire, submerged in the depths of the ocean, and with the requisite speed for the desired purposes? Theoretically, before the experiment of submarine cables was tried, the progress of electricity was supposed to be almost instantaneous. Experience has shown that j when a current of electricity is passes through * a wire covered with a non-conducting body, i ( and that body is surrounded by a conducting ffl& material, a retardation is produced. This is caused by an electrical condition, to which the name induction is given, as we have before stated—the cable with its coverings, becoming something like an extended Leyden jar. It matters little in practice whether the conducting covering outside the gutta percha be an iron wire sheath, the sea, or the earth by which it may be surrounded. In each case the phenomenon of induction is found to retard the passage of the electrical signal. The retardation takes place whether the cable be coiled en masse, or laid straight and covered by earth and water. At present no one can predicate positively how much greater the retarding influences in the depths of the ocean will be than those created in the coils of the electric cable. Many assert that they will be less, and instance the fact of signals urged by the same battery being transmitted with greater speed through the wires of the Mediterranean cables from Sardinia to Malta, and from Malta to Corfu, when laid straight in the depths of the sea than when coiled on board ship. The distances in these cases are short, however, when compared to the enormous length of the Atlantic cable, and the results mentioned cannot be taken as a guarantee of similar effects being produced upon it. From experiments made by Mr. Whitehouse through one thousand miles of the Atlantic cable, it was found that about half a second intervened between making a signal at one end and its appearance at the other ; and it was thought the difficulty arising from the detention of electricity was overcome. It appears, however, from experiments made through the entire cable a short time ago, that a considerable obstacle to rapid communication arises from that cause. It is asserted that these experiments developed the fact that about two words and a-half per minute only can be transmitted through the entire length of wire, which is about one-sixth the speed with which messages can be sent with the present arrangement of symbols from London to Paris.