When I see you in London I will mention to you a piece of secret history which will show you how important our connection with this work may possibly become.
Yours affectionately T.B.M.
The "piece of secret history" above referred to was beyond a doubt the commencement of Macaulay's connection with the Edinburgh Review.

That famous periodical, which for three and twenty years had shared in and promoted the rising fortunes of the Liberal cause, had now attained its height--a height unequalled before or since--of political, social, and literary power.

To have the entry of its columns was to command the most direct channel for the spread of opinions, and the shortest road to influence and celebrity.

But already the anxious eye of the master seemed to discern symptoms of decline.

Jeffrey, in Lord Cockburn's phrase, was "growing feverish about new writers." In January 1825 he says in a letter to a friend in London: "Can you not lay your hands on some clever young man who would write for us?
The original supporters of the work are getting old, and either too busy or too stupid, and here the young men are mostly Tories." Overtures had already been made to Macaulay, and that same year his article on Milton appeared in the August number.
The effect on the author's reputation was instantaneous.