PASSAGE OF RIVERS.

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enemy is also an essential consideration. Should the passage form one of the mainroads for the supply of the army, or the line of retreat in case of any retrogrademovements, great care must be taken to secure it from any possible disaster.* In thesecond case, considered as a mere tactical operation, the passage of a river willprobably be for temporary purposes for the moment or for a few days, when themeans required may be slight, and the celerity of the operation of the first im-portance, the defence of the passage becoming unnecessary. All well-organizedarmies are furnished with a bridge equipment to each corps, at least, with one forthe advanced guard; but whether the passage is formed of materials obtained on thespot, or by a bridge equipment, they are usually removed after the passage iseffected. Up to this date (1850) it does not appear that the means of passingrivers have been brought to perfection, nor are bridge equipments of a sufficientlyportable nature to accompany corps and detachments in rapid movements in difficultcountries. Field Bridges are still cumbersome, and a large establishment of menand horses is required to transport the equipment. In the British Service, in fact,there is no fixed establishment for effecting the passage of rivers.

9. Rules for ascertaining the Width of a River ; 1st, without Instruments .

To ascertain the inaccessible distance a x (see fig. 1), measure any convenientlength a c or a c' in the prolongation of a x; and if the ground wall admit of aline being laid down perpendicularly to the right or left of a, measure the trianglesabc or a b' c', making the sides in the following proportions; viz. a b = 4,a c = 3, and c b = 5, and prolong a b to any point n; then lay down n o inthe same way perpendicularly to a d, and fix a mark at a point in it which is inthe prolongation of b x.

Fig. 1. Fig. 2.

Then, as b d : b a : : D 0 : A x.

Or (see fig. 2) measure ad in any convenient direction, and make o c equal toit, also measuring n o equal to a c, and then fix a mark at b in the prolongationof d x and o c.

Then, asBO:DA::DO:AX.

* Sec ‘Tfcte du Pont.*