A very large number of mules has also been issued to this army for transportation purposes. These mules have been used in making up new wagon trains, and in replacing horses withdrawn from the wagon trains for use of the artillery or cavalry.

I do not understand how General McClellan has fallen into such an error as to the number of horses issued.

The efforts made by this department to supply horses can be understood from the above statement. Advertisements issued at this depot have provided a large number. One thousand were purchased at Harrisburg, without advertisement, about the time General McClellan marched from this city.

Orders were given to the quartermaster at Indianapolis to contract for a supply in that market, to be forwarded to this depot, and 400 or 500 a week have been received from that city. This supply continues, and will continue until the demand ceases, unless the credit of the department fails.

The issues of the 9th, 10th, and 12th of October amounted to 1,578 horses and 711 mules. Two hundred and twenty horses were received here yesterday, and 396 are on hand this morning for issue.

No effort will be spared by this department to supply all the horses necessary to the efficiency of the Army. The number issued to the Army of the Potomac since the battles of the latter part of August exceeds any estimate presented to this department from the staff of that army. The waste and destruction it is perhaps proper that the attention of the Secretary should be directed to the causes of this destruction.

No doubt a number of horses have been killed in action, and some have been captured by the rebels, but in a campaign of no great duration, in a country not a desert, with two railroads leading directly to the field of operations, the number of horses disabled and broken down is alarming.

There are to-day in this depot 2,671 unserviceable horses, broken down by hard usage, by insufficient food and care. The number of such in depot here has been sometimes during the past few weeks as high as 3,300. Many of these horses die, some are shot, some sold as not worth the cost of keeping, but many of them, after a week or two of rest and good feed, recover condition, and are issued again to the army.

The cost of the horses issued within the last six weeks to the Army of the Potomac is probably not less than $1,200,000. The department has purchased these horses on credit. It has to-day, lying in the Treasury, requisitions unfilled which were required in July to meet the demands of the service, and the total amount of its requisitions which have passed the Ward Department, and which still lie in the Treasury unfilled, is $11,334,324.84.

The destruction of horses is a heavy drain upon the military resources of the country, both in horses and money.

I have no report showing the destruction in battle, or the losses by captures, of horses and other supplies of the Quartermaster's Department, during the campaign, but the large number of broken down horses turned into this depot, and requiring to be replaced, indicate a fault in the management of horses in the army.

This is a question of discipline which this department cannot control. The horses of cavalry and artillery, once issued, pass entirely under control of the military commanders. In the wagon trains the enforcement of proper care of the horses and mules also depends upon the state of discipline among the troops.