Harrisburg, July 30.

— A conversation with a gentleman attached to one of the newspapers at Chambersburg, who left there at 8 o'clock this morning, reports the following facts: The people of Chambersburg received information yesterday of the appearance of the rebels. At once the merchants and business men commences packing up their stocks, and before ten last evening most of it was sent far beyond the reach of the rebels.

At 3 o'clock this morning a rebel force of 800 mounted infantry reached Chambersburg, and encamped on the Fair Grounds. This force was met at a distance from Merrsburg by a small party of troops from Carlisle barracks, who opposed the advance stubbornly, but were compelled to withdraw on account of the overwhelming number of the invaders.

The disposition of the rebel forces as they crossed the Potomac is described by our informant as follows: One party came over via Merrsburg, another through Waynesboro, and anyther by Greencastle.

Gen. Averill is reported as having engaged their center, and is said to be falling back on Chambersburg. If this be so his position must be perilous, and he is likely to be surrounded at any moment should the rebels leave Chambersburg to meet him, and those moving from Waynesboro also join in the assault.

The rolling stock of the Cumberland Valley Railroad is being removed to this point.

A large number of fugitives and colored population are in this State, and present a deplorable condition, as they are paraded on the platforms and sidewalks around the railroad depot.

It is reported that the rebels have burned the court house, town hall and the residence of Col. A. K. McClure, at Chambersburg.

Various estimates are made of the number of the invading rebels, the lowest putting them at 85,000.

At 1 o'clock to-day they had made no advance beyond Chambersburg, and our forces still hold Shippingburg, eleven miles east of Chambersburg.

Should the rebels advance on Shippingsburg the movement would develop an apparent intention to make a demonstration still further down the valley, if not to lay siege to Harrisburg.