It would depend upon how quickly you wanted it to be demolished. If time is not important then wooden bridges could be set fire to or just the main timbers sawn but neither of these are practicable if your own infantry are running back across the bridge, hotly pursued by the enemy. In this occasion, which tends to be the most common one, then an instant explosion is most effective. Most armies only destroy bridges during a retreat.

Rafe asked the question and I assumed he was probably talking about the Peninsula, in which case most of the medium sized bridges were, and still are, stone (often Roman constructed) with arches that can only be destroyed by explosions below them. Any attempt to destroy them from above merely forces the locking keystone in the centre even more firmly in place.

A modern demolition team for such a medium sized bridge would probably comprise a section of a 8 men commanded by a Corporal, but they would use relatively small quantities of high explosive and be able to fix it quickly using bolt guns. I would guess that in the Napoleonic era the same bridge would require a quarter-company section of some 15-25 men, commanded by a sergeant, due to the amount of gunpowder needed to be placed and the complications of constructing a secure method of attaching the explosive to the bridge. In those days it may have required an officer to calculate the amount of explosive needed and the placement of the charges. Such matters were taught to officers at military academies in Great Britain and France, whilst NCOs would have picked up a limited knowledge by practical experience. Today, Royal Engineer NCOs are trained in such calculations.

I have a copy of the First World War "Manual of Field Engineering - 1911", and this covers many of the basic techniques which would have been in use in the Napoleonic Wars. Chapter 14 is entitled "Hasty Demolitions with Explosives" and included demolition with Guncotton and Dynamite, neither of which would have been available in the Napoleonic era. It also included the calculations for using Gunpowder, although it says it is not so suitable unless a lifting and shaking effect is required due to its [relatively] slow rate of burning. It says that gunpowder shuld be well tamped, probably in sandbags of about 40 pounds each. The formula to attack a masonry arch is that the number of pounds of gunpowder required = 2/3BT(squared) where B = length to be demolished in feet and T = thickness to be demolished in feet. It also says that in the presence of the enemy, charges may be placed hurriedly and so should be increased by 50%. The charges are divided equally and placed at distances apart equal to twice the thickness of the object to be demolished. If a narrow Napoleonic stone bridge was 10 feet wide and 4 feet thick it would thus require at least 730 pounds, possibly 1,100 pounds per cut, this being some 20 to 30 charges each of the 40 pounds capable of being lifted by one man. Normally the bridge would be cut in two places so as to drop a span and this would double the amount of gunpowder needed.

Rod