For reconnoissance work the large bi-motor Caudrons are generally used. They are fitted with a small wireless apparatus; but this means of communication cannot be used very often. The machines, on many occasions, have to go beyond the effective radius of their radio, and at other times its use is inadvisable, as its messages might become known, or else blocked by the enemy. Resort has been had, therefore, to carrier-pigeons. These are released the moment any important information has to be conveyed to headquarters, and these swift little messengers have proved extremely useful and reliable. Their use has, in consequence, become general. The bimotor Caudrons are employed also by the photography section of the army, though much of this work is actually done by artillery machines detailed for this service. Photography is a dangerous duty, because the flights have to be made at low altitudes to obtain the best results. On the other hand, it is not at all tedious. The mission upon which the machine has been sent is usually accomplished in a brief space of time, and the machine often stays out less than an hour. In comparison with the amount of flying required of the aviators in the other branches of the service, which varies in length from three to five hours a day, photography is easy work. "Réglage," or fire-control, on the other hand, is the most difficult and the most dangerous work to be performed by the Flying Corps at the front. The machines used are large and unwieldy, built to carry the weight of two men and all sorts of equipment. They are fairly fast, but their spread of wing is so large that it is almost impossible for them to make a turn quickly when attacked. They are armed with a machine gun, it is true, but they are always at a great disadvantage in the presence of an enemy fighting-machine which can out-manoeuvre them at every turn.