MY LORD,--We have the honour to acquaint your Lordship that the army marched from Ghuzni, _en route_ to Cabool, in two columns, on the 30th and 31st ult., his Majesty Shah Shooja-ool-Moolk, with his own troops, forming part of the second column. On the arrival of the Commander-in-Chief with the first column, at Hyde Khail, on the 1st inst., information reached him, and the same reached the Envoy and Minister at Huft Assaya, that Dost Mahomed, with his army and artillery, were advancing from Cabool, and would probably take up a position at Urghundee or Midan, (the former twenty-four, the latter thirty-six miles from Cabool.) Upon this it was arranged that his Majesty, with the second column, under Major General Willshire, should join the first column here, and advance together to attack Dost Mahomed, whose son, Mahomed Akhbar, had been recalled from Jellahabad, with the troops guarding the Khyber Pass, and had formed a junction with his father; their joint forces, according to our information, amounting to about 13,000 men. Every arrangement was made for the King and the army marching in a body from here to-morrow; but in the course of the night, messengers arrived, and since (this morning) a great many chiefs and their followers, announcing the dissolution of Dost Mahomed's army, by the refusal of a great part to advance against us with him, and that he had in consequence fled, with a party of 300 horsemen, in the direction of Bamian, leaving his guns behind him, in position, as they were placed at Urghundee. His Majesty Shah Shooja has sent forward a confidential officer, with whom has been associated Major Cureton, of her Majesty's 16th Lancers, taking with him a party of 200 men and an officer of artillery, to proceed direct to take possession of those guns, and afterwards such other guns and public stores as may be found in Cabool and the Balla Hissar, in the name of, and for his Majesty Shah Shooja-ool-Moolk, and the King's order will be carried by his own officer with this party, for preserving the tranquillity of the city of Cabool. A strong party has been detached in pursuit of Dost Mahomed, under some of our most active officers. We continue our march upon Cabool to-morrow, and will reach it on the third day. We have, &c., (Signed) JOHN KEANE, Lieut.-General, Commander-in-Chief. (Signed) W.H. MACNAGHTEN, Envoy and Minister. * * * * * _Extract from a Letter from his Excellency Lieutenant-General Sir John Keane, K.C.B. and G.C.H., dated Head-Quarters, Camp, Cabool, August 8th, 1839_:--