Let us take first its uses in connection with the Navy. The aerial wires or antenna are stretched to the top of the highest mast of the vessel. Where there are two masts they often span between the two. Ships which have masts for no other reason are supplied with them for this special purpose. In the case of submarines, the whole thing, mast and wires included, is temporary and can be taken down or put up quickly and easily at will. The stations ashore are equipped much after the same manner as are the ships, except that sometimes they are a little more elaborate, as they may well be since they do not suffer from the same limitations. For example, the well-known antenna over the Admiralty buildings in London consists of three masts placed at the three corners of a triangle with wires stretched between all three. However these wires may be arranged and supported they are very carefully insulated from their supports, for when sending they have to be charged with current at a high voltage and need good insulation to prevent its escape, while, in receiving, the currents induced in them are so very faint that good insulation is required in order that there may not be the slightest avoidable loss. The function of these wires, it will be understood, is to form one plate of a condenser, the earth being the other plate and the air in between the "dielectric" or insulator. In the case of ships "the earth" is represented by the hull of the vessel. It makes a particularly good "earth" since it is in perfect contact with a vast mass of salt water, and that again is in contact with a vast area of the earth's surface. Salt water is a surprisingly good conductor of electricity. In land stations "earth" consists of a metal plate well buried in damp ground. The whole question of conduction of electricity through the earth is very perplexing. There seems to be resistance offered to the current at the point where it enters the ground, but after that none at all. Consequently the resistance between two earth plates a few yards apart and between similar ones a thousand miles apart is about the same. Though the earth is made up mainly of what, in small quantities, are very bad conductors indeed, taking the earth as a whole it is an exceedingly good conductor. That makes it all the more important that where the current enters should be made as good a conductor as possible, and the construction and location of the earth plates is therefore very carefully considered so as to get the best results.