If we refer to experience the following are the results, which may serve as a basis in any calculations of this kind.
A division of ten or twelve thousand men, with a proportion of cavalry, a day’s march of three or four miles in advance in an ordinary country, not particularly strong, will be able to detain the enemy (including time occupied in the retreat) about half as long again as he would otherwise require to march over the same ground, but if the division is only a mile in advance, then the enemy ought to be detained about twice or three times as long as he otherwise would be on the march.
Therefore supposing the distance to be a march of four miles, for which usually ten hours are required, then from the moment that the enemy appears in force in front of the advanced corps, we may reckon upon fifteen hours before he is in a condition to attack our main body. On the other hand, if the advanced guard is posted only a mile in advance, then the time which will elapse before our army can be attacked will be more than three or four hours, and may very easily come up to double that, for the enemy still requires just as much time to mature his first measures against our advanced guard, and the resistance offered by that guard in its original position will be greater than it would be in a position further forward.
The consequence is, that in the first of these supposed cases the enemy cannot easily make an attack on our main body on the same day that he presses back the advanced corps, and this exactly coincides with the results of experience. Even in the second case the enemy must succeed in driving our advanced guard from its ground in the first half of the day to have the requisite time for a general action.
As the night comes to our help in the first of these supposed cases, we see how much time may be gained by an advanced guard thrown further forward.