The Tunnel Blockade to be Raised. - Yesterday, in Company with John Piper, we paid a flying visit to the obstructed tunnel at American Flat. On arriving at the depot we found that the regular train had gone and we were compelled to take passage on a "special," consisting of a hand car manned by half a dozen trackmen. Piper assisted in getting the train under headway, on being assured that its destination was the American Flat Tunnel. On arriving at the lower end of Gold Hill, the proprietor of the hand car informed us that his train was going no farther, and we were compelled to strike out across country on foot. A walk of twenty minutes brought us to the scene of the late conflagration. The following is the result of our observations: The northern entrance to the tunnel has been cleared of fallen rock and timbered for a distance of thirty feet. The workmen appear to be making good progress though laboring under numerous disadvantages. Yesterday afternoon the heat was so intense in the upper portion of the tunnel that the workmen were only able to stay at their posts long enough to drive a single nail, ' when they would be compelled to beat a hasty retreat. Yesterday morning the services of No.2's steamer were again called into requisition, and the heated fragments of rock were thoroughly "hydraulicked." A large number of Cornishmen were busily engaged in blasting and removing the boulders which block up the south entrance to the tunnel. A number of Virginia lads were in charge of the drays, and among the sappers and miners we recognized our friend Bishop, a man of courage and energy, whose services the company are fortunate in having at their command. It is expected that by this evening the south entrance to the tunnel will be opened sufficiently to admit of better ventilation. In view of the uncertainty as to the time required to repair the tunnel, the officers of the company have decided to build a temporary track around the east side of the hill pierced by the tunnel. Over this track the cars will be drawn by horses. To avoid the necessity of making deep excavations, turn- tables will be constructed at both ends of the horse-car track, and at other points if necessary, in order to facilitate the passage of the cars around short curves. A large gang of men will commence the work of grading this morning. Mr. James, the engineer is sanguine of his ability to have the cars running over the new track inside of ten days. Meanwhile, the work of repairing the tunnel, under the management of Superintendent Holmes, will be pushed forward vigorously.