Telephones and all other means of transmitting intelligence rapidly are now of the utmost importance to both attack and defence. Maps marked with numbered squares are necessary for directing artillery fire, especially from cupolas. Organization in every branch will give better results than ever before, and the question of communication and transport from the base of supplies right up to the front needs detailed study, in view of the great weight of ammunition and supplies that will have to be handled. The use of light mortars for the trenches, introduced by Coehoorn and revived with extemporized means at Port Arthur, needs great attention. It may be prophesied that the issue of important sieges in the future, when skilfully conducted on both sides with sufficient resources, will depend mainly on the energy of the defenders in trench work, on mining and countermining in connexion with the trenches, and on the use of light mortars made to throw large charges of high explosive for short distances with great accuracy. For a brief narrative of the siege of Port Arthur in 1904, one of the greatest sieges of history, both as regards its epic interest and its military importance, the reader is referred to the article RUSSO-JAPANESE WAR. DEFINITIONS.--The following definitions may be useful, but have no place in the evolution of the attack, to which this section is mainly devoted. _Investment._--This most necessary, almost indispensable operation of every siege consists in surrounding the fortress about to be besieged, so as to cut off its communications with the outside world. _Preliminary investment_ which is carried out by cavalry and light troops before the arrival of the besieging force, consists in closing the roads so as to shut out supplies and reinforcements. _Close investment_ should be of such a character as to prevent any sort of communication, even by single messengers or spies. The term "_blockade_" is sometimes loosely used instead of investment. _Lines of Circumvallation and Contravallation._--These now obsolete terms were in great use until the 19th century. The _circumvallation_ was a line of parapet which the besieger made outside the investing position of his own force, to protect it when there was a chance of attack by a relieving army. The line of _contravallation_ was the line of parapet and trench sometimes made by the besieger all round the town he was attacking, to check the sorties of the garrison. _Observing Force._--When circumstances make the reduction of a particular fortress in the theatre of operations unnecessary a force is often detached to "observe" it. The duty of this force will be to watch the garrison and prevent any hostile action such as raids on the lines of communications.