From Petersburg.
Petersburg, August 18.
--About two o'clock this morning our batteries along the whole line in front of this place opened fire upon the enemy, who responded feebly.
The firing was kept up for an hour and a half, when all subsided into usual quiet on our side.
No casualties are reported.
The enemy's cavalry are making some demonstration on our right this morning.
[Second Dispatch.]
Petersburg, August 18.
--A force of the Fifth corps of the enemy, assisted by cavalry, broke through our cavalry lines on our right this morning, and succeeded in reaching the Weldon railroad just above thesix-mile station, tearing up a part of the railroad — some say as much as two miles. H. H. Walker's Virginia and Davis's Mississippi brigades coming upon the raiders, a sharp fight ensued, the result of which is not fully ascertained.
One hundred and fifty-eight prisoners, including eight officers, captured in the fight, have arrived here.
The enemy is in retreat, by latest advices.--The prisoners say the move had a two-fold object, to draw troops from our front in order, if possible, to charge our works, and, secondly, to break the road. 