The great depth of a detachment of ski-runners is a disadvantage. Each ski-runner takes up 2.20 m., and in addition a distance of 1 m. must be left between men. Besides, as the men are not all equally skilled in using skis, the road space taken up by a detachment of runners is increased considerably. This necessitates the employment of small detachments not exceeding 100 men. The proper sphere of ski-runners is patrol duty. In winter campaigns ski-runners are a necessary substitute for cavalry. In Scandinavia reconnaissance detachments are composed of ski-runners and cavalry, the horses pulling the runners. This permits a very rapid movement over snow-covered ground and may also be employed where the use of the much slower sled is impracticable owing to the nature of the terrain. Much practice is undoubtedly necessary to acquire proficiency in this sport[32], for it surely cannot be easy to preserve an upright position on skis behind a galloping horse, while at the same time overcoming all the difficulties of the ground. When troopers cannot continue to perform reconnaissance work, they are replaced by snowshoe or ski-runners. The snowshoe performances in France, since the establishment by War Department order of a training school in Briançon, are worthy of note. According to the experiences gained at that institution, a period of four weeks is ample for training men for extended mountain marches. On January 18th, 1904, a body of men on snowshoes covered a distance of 80 km., including a climb of 1,700 m., in 20 hours (4 hours’ rest included). [32] During the Norse winter sports of 1905, the winner covered a distance of 67 km. in 2³⁄₄ hours. 2. THE TACTICAL UNIT. By the term tactical unit is meant the smallest element of a body of troops capable of sustaining an action independently, of performing a simple combat task, and the elements of which (man and horse) are personally known to the leader. Moreover, the tactical unit should be small enough to allow of its being controlled by the voice of a single leader. This requirement limits the battle front of the tactical unit to about 100 m., which corresponds approximately to the front of a troop[33] (_Eskadron_) in line, or to that of a battery of six pieces. The frontage of the tactical unit of cavalry might with advantage be decreased, while that of artillery, on account of its stability in action, might be extended, were it not for the fact that the increase in the quantity of matériel and in the number of horses involved in such extension would make the supervision of the unit too difficult for a single person. In nearly all large armies the strength of a troop (_Eskadron_) of cavalry and of a field battery is approximately 150 horses.[34] This corresponds approximately to what the farmer of northern Germany considers suitable to keep together in one establishment. If the estate is larger, requiring more than 150 men and horses, subsidiary farms[35] are established.