An official dispatch, received yesterday, states that Sheridan's forces encamped on Thursday night at New Market, in Spotsylvania county, about five miles from Childsburg, and ten miles south of Spotsylvania Court-House. We have good authority for stating that Gen. Hampton was between them and Gordonsville. A citizen of Caroline county, who arrived in this city yesterday, states that Sheridan's command is about , with ten pieces of artillery, and that they moved in three columns. Other , however, represent the force as considerably greater in numerical strength.

A rumor was in circulation yesterday that Hampton had surprised the enemy's camp and put them to rout, but this lacks confirmation. We only know with certainty that some prisoners had been captured, who were seen by a scout yesterday on the way to Richmond.

We have received some further intelligence from the Valley. Crook and Averill joined Hunter on Wednesday at Staunton, at which time a portion of their forces were on the Greenville and Middlebrook road. On the same day, at 3 o'clock P. M., five hundred cavalry made a demonstration on Waynesboro', on the Greenville and Staunton road, and were repulsed by Gen. Imboden. The enemy retreated to Staunton, burning the Fishersville Depot on the route.

Pope, with a force of 4,000, was reported moving down the Valley to reinforce Hunter. The enemy have no supplies, but subsist off the country. They were doubtless aware of richness of the region into which they have penetrated, and will strip the inhabitants of everything they possess.

A dispatch from Mountain Top says our troops are in fine spirits, and anxious to measure strength with the enemy. The Yankee cavalry made another advance on Thursday, but were easily repulsed by Imboden's infantry.

All quiet in Petersburg yesterday. The enemy have retired in the direction of City Point. Official dispatches give great praise to the militia for their gallantry in repulsing three assaults, and only giving way in pursuance of orders, after they were flanked.

The Petersburg Express of yesterday contains the following account of the attack and repulse of the enemy. We give it in full.

I will be recollected that we stated in Wednesday's issue, when noticing the skirmishes which occurred on two roads in Prince George county, the evening previous, that they were the precursors of more for demonstrations, which in all probability would soon threaten us. Our opinion proved correct, and the formidable demonstrations were not long delayed.

Yesterday morning, at 1 o'clock, three distinct discharges of cannon were heard in the direction of the enemy's whereabouts, and at early dawn pickets were driven on the City Point and Prince George Court-House roads. It was subsequently ascertained that these movements were but into to deceive our forces, while the real movement for the surprise and capture of the city was on the Jerusalem Plank road, coming into Petersburg from a southerly direction. On the two first named roads the enemy appeared in considerable number as early as seven o'clock, and brisk skirmishing was kept up for some time. At eight o'clock the Court house and engine bells were rung, to which the citizens responded with their usual alacrity, and manifested every disposition to defend their homes and firesides.

In the meantime the enemy's gunboats ascended the Appomattox river and opened a furious fire on Fort Clifton, and various other points along the river, for the obvious purpose of occupying the attention of our troops in Chesterfield.

At 9 o'clock our pickets on the Jerusalem plankroad were driven in, and before ten the enemy showed himself in overwhelming number filling the road and the woods on both sides. Our breastworks here extended from the residence of Timothy Rives, Esq., on the left, to and across the road and beyond the house of Mr. Wm. A. Gregory, on the right. The enemy manœuvred for a while, thinking probably that our raw troops fight, but never were the Yankee invaders more mistaken. Our men were made of sterner stuff, and inspired by the cool determination of their leaders, Gen. Colston and Col. F. H. Archer, maintained their ground like veterans. Finally the enemy ordered a charge, and came down to our breastworks with a yell, their drawn sabres flashing in the sunlight. When within fort paces of the fortifications the order to fire was given, and the Yankees recoiled and fell back. A prisoner, taken subsequently, states that in this charge the notorious Speare led, and that they had forty wound and two killed. This charge was repeated twice, but with like results, when the enemy resorted to the flanking process, which, by reason of his overwhelming numbers, he was enabled to do with much ease. A short time afterwards a regiment came around Rives's house on our left, another appeared on our right, and a large body came down in front. We had but one hundred and seventy men all told, and it was impossible with this number to guard centre, right, and left, along, a length of three quarters of a mile of more. The order was given to retreat, and in a few minutes the enemy had possession of our works, our came, and were in full pursuit of our men. Couriers had been dispatched for reinforcements, but they did not come up in time to save our fortifications, and many of Petersburg's best and most gallant sons fell in the affray, some killed and others wounded.

The enemy came on in double column, with sabres drawn, until they reached the hill opposite the water works, where they planted a cannon for the purpose of shelling the city. They then started down the hill, and their column actually took possession of a bridge which crosses Powell's Run at the foot of the Water Works Hill.

They were almost in Petersburg — could see its spires and steeples and many of the houses on our suburban limits — but again that Divine arm which has been so often outstretched in our behalf, was bared, and our city was saved from the tread of the ruthless invader. Just at this opportune moment Graham's battery reached the Reservoir Hill, unlimbered in an instant, and with a precision and rapidity which we have heard spoken of as being almost without precedent, threw into the ranks of the enemy a shower of shell. The missiles of death coming so unexpectedly to the foe, he at first seemed overwhelmed with surprise, and halted, neither advancing nor retreating. But a minute or two later another branch of our service made its appearance, which quickly determined the enemy as to the best course for him to pursue. Dearing's cavalry brigade quickly dismounted, and descending the hill with a yell, charged upon the enemy in beautiful style. This was more than they expected, (since they had encountered but a few militia in the breastworks, and had advanced nearly a mile without seeing any regulars) and they instantly wheeled their horses and started back up the hill in great confusion. Graham's battery continued to play upon them, and Dearing's men crossed the refine and ascended the opposite hill, in gallant style, their carbines keeping up a regular and most musical fusillade upon Kantz and Speare, and their rapidly retreating followers. Upon reaching the tops of the opposite hill, the enemy hoped to make a stand, as here another column, which they had sent on to the city for the purpose of entering by the Blandford Church road, have in sight. But this column had also started on a retrograde movement, for, to their surprise, too, they had encountered Sturdivant's battery, which had gone out by another road, and the two columns met and continued their retreat at a speed not at all slackened by the Confederate shell and balls which were falling thick and fast among them. In Jackson's field, a mile or so from the Blandford Church, we captured a handsome cannon and six horses, which the enemy were compelled to abandon in their flight. Our forces pursued them for a few miles, killing and wounding many, and taking some prisoners.

Yesterday afternoon late, our pickets extended for several miles out on the Jerusalem plank road, but no signs of the enemy could be seen, and it is supposed they have retreated to the river. This is only supposition, however, and the vandals will bear constant and vigilant watching.

Gen. Kantz was in command of this force, and prisoners taken variously estimate it at from three to five thousand. None put the number at less than three thousand, and some say that it was even more than five thousand. There is no doubt but that it was intended to capture this city, and all the circumstances are strongly corroborative of this view. Thanks to a kind Providence, who has nerved the hearts and strengthened the hands of our brave men, we have been again preserved.

The enemy crept up behind the residence of Mr. Wm. A Gregory, ascended to the roof, and knocking off the shingles were enabled not only to obtain an excellent view, and ascertain the number of our forces, but through the openings thus made fired upon and killed many of our men behind the breastworks.

The residence of Timothy Rives, Esq., fell into the possession of the invaders, after our forces retreated, and the scoundrels not only ransacked and robbed it of all its valuable contents, but then applied the torch, and burnt it to the ground. They also carried Mr. Rives off a prisoner.

The foregoing are substantially all the facts connected with this bold attempt to capture Petersburg, so far as we have been enabled to collect them, but the saddest part of our statement yet remains unrecorded. Petersburg's bravest and best blood flowed freely yesterday, and many a household has bee plunged into the most inconsolable grief.--We witnessed scenes yesterday afternoon, which we trust we may never witness again. All who have been so stricken, have the consolation — a sad one it is true — but nevertheless, it is consoling to know, that those who have been so suddenly snatched from them, fell in a glorious cause. Their memories will ever be held in grateful recollection by our people, and this whole community deeply sympathize with the bereaved. The following is a list of the killed and wounded, which, with great care, we have obtained, and which we believe to be correct:

Prof. Stanbly, Professor of French in the Petersburg Female College, formerly of Randolph Macon.

Lt. Wales Hurt, aged 18, a son of the late Branch T. Hurt, and a gallant youth.

John E. Friend, oldest son of our late Postmaster, Wm. N. Friend, Esq. Deceased leaves a wife and one child.

Wm. C. Bannister, for many years and accountant in the Exchange Bank, and one of our best citizens. He leaves a large family.

Geo. B. Jones, a prominent druggist of Petersburg, and a most excellent man.

John Crowder, of this city, a noble youth.

George R. Conway, of Prince George.

Wm. Daniels, of Prince George, a youth. He was killed by a bayonet thrust through the breast, and his head was severely mutilated with the butt of a musket.

E. P. Brown, of Sturdivant's battery, from Albemarle.

Henry A. Blanks, of this city, seriously in body and arm.

Dr. Wm. Bellingham, of Petersburg, a prominent dentist, severely in side.

Wm. Howard, a clerk at the bookstore of Richard Riches.

Wm A. Meanley, of this city, shot in three places severely.

Robt. A. Martin, of the firm of Martin, Tannahill & Co, shot in the arm.

Wm T. McCandlish, merchant of this city, shot in the hand, and a prisoner.

Richard A. Harrison, of this city, shot through the hand.

Samuel Hall, bat merchant of Petersburg, shot in the thigh, severely.

W. H. Hardee, merchant of Petersburg, in the leg below the knee, since amputated.

Lt. Geo. V. Scoot, of this city, severely in face and leg.

Norbourne T. Page, of this city, in the hand.

Adjutant Guy G. Johnson, shot in the bowels, feared mortally wounded.

Joseph Cooper, son of Joseph H. Cooper, Esq., of this city, wounded and a prisoner.

A. C. Harrison, of this city, slightly in the arm.

Richard M. Cary, formerly of Hampton, but now of Petersburg, in the thighs in two places — not mortal.

C. L. Bartlett, Professor of Music, in three places, severely.

A. S. Shafer, of Petersburg, in leg, slightly.

Nathaniel Pritchard, of this city, slightly.

Wm. F. Johnson, of Albemarle, (Sturdivant's battery,) through the head — supposed mortally.

Wm. Griffith, of Prince George, wounded in both thighs

Jas Kane, of Prince George, in the leg.

Rev. Wm. A. Hall, Chaplain Washington New Orleans Artillery.

Prof Carr, of the Petersburg Female College.

James Bossieux, merchant of this city.

James Kerr, of Petersburg

Timothy Rives, of Prince George.

Samuel Jones, of Prince George--6.

The following prisoners were brought in at an early hour yesterday afternoon:

Wm. Tompkins, co. F., 11th Penn cavalry, commanded by Col. Speare; Solomon Saxton, do; John Logan, co. D., same regiment, and Peter White, bugler, of the same regiment.

These men all agree that the expedition was for the purpose of capturing Petersburg. They say that Gen. Kantz was in command, and that Colonel Speare was along.

They left Bermuda Hundred at half past 10 o'clock on Wednesday night, and crossed on a pontoon bridge three miles this side of City Point.