The Masked Letter is a form of steganography.
Recommended reading: The Codebreakers: The Comprehensive History of Secret Communication from Ancient Times to the Internet.
You will have heard, Dr Sir I doubt not long before this can have reached you that Sir W. Howe is gone from hence. The Rebels imagine that he is gone to the Eastward. By this time however he has filled Chesapeak bay with surprize and terror.
Washington marched the greater part of the Rebels to Philadelphia in order to oppose Sir Wm's. army. I hear he is now returned upon finding none of our troops landed but am not sure of this, great part of his troops are returned for certain. I am sure this countermarching must be ruin to them. I am left to command here, half of my force may I am sure defend everything here with much safety. I shall therefore send Sir W. 4 or 5 Bat [talio] ns. I have too small a force to invade the New England provinces; they are too weak to make any effectual efforts against me and you do not want any diversion in your favour. I can, therefore very well spare him 1500 men. I shall try some thing certainly towards the close of the year, not till then at any rate. It may be of use to inform you that report says all yields to you. I own to you that I think the business will quickly be over now. Sr. W's move just at this time has been capital. Wahingtons have been the worst he could take in every respect. sincerely give you much joy on your success and am with great Sincerity your (?)
Sir. W. Howe is gone to the Chesapeak bay with the greatest part of the army. I hear he is landed but am not certain. I am left to command here with too small a force to make any effectual diversion in your favour. I shall try something at any rate. It may be of use to you. I own to you I think Sr W's move just at this time the worst he could take. Much joy on your success.