It explains itself.
26th September, 1803.
TO CAPTAIN SIR RICHARD JOHN STRACHAN, BART., H.M.SHIP DONEGAL.
The occurrences which pass every day in Spain forbode, I fancy, a speedy War with England; therefore it becomes proper for me to put you on your guard, and advise you how to act under particular circumstances.

By looking at the former line of conduct on the part of Spain, which she followed just before the commencement of the last War, we may naturally expect the same events to happen.

The French Admiral Richery was in Cadiz, blocked up by Admiral Man; on August the 22nd, they came to sea, attended by the Spanish Fleet, which saw the French safe beyond St.Vincent, and returned into Cadiz.

Admiral Man very properly did not choose to attack Admiral Richery under such an escort.
This is a prelude to what I must request your strict attention to; at the same time, I am fully aware that you must be guided, in some measure, by actual circumstances.
I think it very probable, even before Spain breaks with us, that they may send a Ship or two of the Line to see L'Aigle round Cape St.Vincent; and that if you attack her in their presence, they may attack you; and giving them possession of the Donegal, would be more than either you or I should wish, therefore I am certain it must be very comfortable for you to know my sentiments.

From what you hear in Cadiz, you will judge how far you may venture yourself in company with a Spanish Squadron; but if you are of opinion that you may trust yourself near them, keeping certainly out of gun-shot, send your Boat with a letter to the Spanish Commodore, and desire to know whether he means to defend the French Ships; and get his answer in writing, and have it as plain as possible.