We have received a communication on this subject from A. Hall, of Washington, D. C, and as we have not room for it in extenso, we will present its substance. He states that one most important point has been entirely overlooked in all the experiments made with the cable up to the present time, regarding its operative powers if laid in the Atlantic. In the reports published we are told that such and such a person has succeeded in sending dispatches through the entire length of cable, and that Professor Hughes has transmitted three or four words per minute ; and the company and the scientific wo/ld are congratulated upon the electrical part of the work being ensured. The point to which Mr. Hall wishes to direct special attention is, that in all the experiments thus made to send messages through the entire cable, the two ends of it have been brought together, which makes but one-half of the circuit when the two ends are two thousand miles apart, as the water of the Atlantic must form one-half of the circuit (the return) when the cable is laid. "All the experiments made," he says, "have been based upon the assumption that there will be nothing but the cable to operate through when stretched across the ocean, whereas every electrician knov/s that the water forms the return stretch; and the electric wave must not only travel the entire length of the cable, but must return through the water, which is not so good a conductor as copper by 33 per cent. It therefore follows that the return part of the electric circuit is equivalent to adding three thousand miles to the cable, making a total of five thousaid miles of a circuit. Until experiments are made to compensate for the water part of the ^ circuit, it is premature to claim the problem of telegraphing through the Atlantic as solved, especially as it is with extreme difficulty they have been able to send reliable signals through j less than one-half the circuit." | Mr. Hall is correct in his statements, so far ^ as we can recollect, in reading the accounts of the experiments to which he refers ; but the London papers may not have given full and correct descriptions. Considering the amount of talent engaged in these experiments, it will be surprising if the important point alluded to has been overlooked. We have seen accounts of our countryman, Mr. Hughes, havmg been more successful than any of the English electricians in operating the cable, but how his success was obtained has not been explained. If this telegraph cable should be successfully laid, and if only two words can be sent through it per minute, it will but be the pioneer of other lines ; and such improvements will be made, we doubt not, but that messages will be ultimately sent through the ocean with much greater rapidity. It has been stated that it will require a battery of no less than five hundred cells to operate the Atlantic cable.