Artillery, however, is incapable of independent action--it must always be associated with the other Arms, as it is easily avoided or turned, and, when moving, is helpless against attack. It takes up a great deal of space in the column of march, as well as on the battlefield, where it requires advantageous positions to fire from, and cover for its horses and ammunition, both often difficult to find. Artillery is also very dependent on the weather and the nature of the country for its action, as it requires clear air and good light, and an absence of hills and woods, to allow the object and the effect of its fire to be observed. It also needs good roads, and is more obstructed by mud, ice, or snow on the march than are the other Arms. 3. ENGINEERS Engineers, as a body of officers with men, were only introduced towards the end of the eighteenth century, but officers of that name had been employed for centuries on the Staff of Armies, especially at Sieges. The Engineers are now sometimes styled “The Fourth Arm of the Service,” not so much because they are Combatant Troops, armed and trained like Infantry, as because their work on the battlefield is of interesting tactical importance. The work with which Engineers with an Army in the Field are charged presents great scope and variety. It may be catalogued under the following headings: (_a_) WORK WITH THE FIGHTING TROOPS Pioneer Work on the march--_i.e._ making roads and removing obstacles; water supply; bridging of every sort; collecting, making, and using boats and rafts for ferrying. Field Work on the battlefield--_i.e._ clearing the communications and field of fire; marking ranges; demolitions; obstacles; special earth-work (ordinary trench-work and gun-pits being made by the troops who use them). Searchlights in the field. Inter-communication Work--_i.e._ use of telegraphs, telephones, wireless, visual signalling, kites, captive balloons. Aviation by balloon or airship. Printing and lithography for Orders and Maps. (_b_) WORK IN REAR OF THE FIGHTING TROOPS Engineers are also charged with the following important work on the Lines of Communication: Construction, repair, maintenance, and working of railways and telegraphs; provisional fortification of posts; camping grounds; formation of workshops and depôts of Engineer Stores; hutting and housing troops; providing hospitals, offices, and storehouses; water supply; roads. At sea bases, piers, wharves, and tramways will have to be provided, and perhaps dredging undertaken, and buoys, beacons, and lighthouses kept up. Engineers will also have to run any _plant_ needed, such as that for providing ice for hospitals, cold storage, electric light and power, gas for balloons and lighting. Engineers are employed in surveying, or mapping the country passed through by the Army, when this is required in the wilder theatres of operations, like the Indian Frontier.