fixed on the north side of the iron band in which the large points are fastened; the other end of this wire is left free, and can be utilised as a conductor for a rheometer or any other electric machine which it might be thought proper to use permanently for the registration of lightning striking the conductors. The eight conductors have each an unbroken continuity of about 310 feet; and they collectively show a continuous section of nearly one inch--almost half as much again as the limit of safety given in the ‘Instruction’ of the Paris Academy. Although, in Professor Melsen’s opinion, rods of somewhat less diameter would have been amply sufficient for security, he chose the largest size which could be easily bent to the varying contours of the building, and also as allowing for the expansion and contraction caused by changes of temperature. If conductors of only one quarter of an inch diameter had been used they would, it is true, have shown a total section just above the limit of the ‘Instruction;’ but, since Coulomb has demonstrated that tensional electricity is more particularly carried on the surface of bodies, M. Melsens thinks it is necessary to consider the action that this surface might exercise in the easy transmission of electricity. Some old German writers on this subject went so far as to assert that the conductivity was proportioned to this surface. They therefore recommended flat bands or hollow tubes in place of rods. Although exact figures cannot be given as to the effect due to the area of the surface, M. Melsens considers that it is unquestionable that the relation of the section to the surface has a marked and definite, although at present unknown, result. In the case of the Hôtel de Ville, Brussels, he thinks the eight conductors possess a signal advantage over one conductor, even though it had a larger section--say one inch. Experience will doubtless teach how to determine more precisely the extent of this surface-action.