The observation and maps thus made were of the greatest importance, and readily enabled the commanding officer to decide what course he would pursue.

In the evening of the same day I received the following order from General McClellan:

HEADQUARTERS ARMY OF THE POTOMAC,
April 6, 1862.

Professor LOWE:

General McClellan directs that you send a balloon to General Keyes" headquarters at Warwick Court-House as soon as possible.

By command of Major-General McClellan:

A. V. COLBURN,

Assistant Adjutant-General.

In compliance with this order I proceeded to Fortress Monroe to move another balloon to General Keyes" command, and left the one then inflated and in use before Yorktown in charge of the only assistant aeronaut I was then allowed, excepting one in charge of the balloon-boat at Fortress Monroe.

After stationing the balloon at Warwick Court-House (the train having to move over the worst roads I ever saw) I started on the night of the 10th for Yorktown. Our lines having been changed during my absence, I found myself, about 9 o"clock p.m., within the enemy's lines. I was not sensible of the danger I was in until I heard signals given by a low whistle, which I at once knew to be those of the rebels, and accordingly cautiously retraced my steps and spent the night at the camp of one of our advanced regiments. The next morning at daybreak I took the road to Yorktown, and at 6.30 I was surprised by the descent of a balloon very near me. On reaching the spot I found it to be the one I had left in charge of my assistant at Yorktown, and General Fitz John Porter the occupant. The gas had entirely escaped when the balloon reached the earth, from the fact that the general in his eagerness to come to the ground (on finding that the rope by which the balloon was let up had parted) had opened the value until all the gas had escaped, and as the balloon was constantly falling the silk was kept extended, and presented so large a surface to the atmosphere that it served the purpose of a parachute, and consequently the descent was not rapid enough to be dangerous.

I would here remark that a balloon suddenly relieved of its gas will always form a half shpere, provided it has a sufficient distance to fall in to condense a column of air under it. A thousand feet would, I presume, be sufficiently high to effect this and to make the descent in safety.

On inquiring into the cause of the accident I found that Mr. Allen, the assistant in charge of the balloon, had used but one rope, as had used but one rope, as had been his idea of topical ascents, instead of three and sometimes four, as I always did, and that rope had been partially injured by acid which had accidentally got on it.

I found it difficult for a time to restore confidence among the officers as to the safety of this means of observation on account of this accidentpersonal ascensions I made gradually secured a return of their favor, and on the 13th of April I received the following communication:

APRIL 13, 1862.

PROFESSOR: General Barnard is General McClellan's chief engineer, and is located in his camp. General McClellan is very anxious for him to have an ascension early in the morning, and General B. will be prepared to accompany your messenger, whom I beg of you to direct to wait to take General Barnard to