The Swedish engineer, Virgin, (1781,) a man of genius and practical ability, proposed fronts of 400, 500, and 550 yards; he makes use of a tenaille with flanks to bring a musketry fire on the salients; this is necessary, especially, in the large fronts, in which the line of defence is 330 yards, counting from the bastion flanks to the opposite salient; in this front the tenaille flanks, (which form a salient or redan in front of the centre of the tenaille,) are within 280 yards of the same salients.