Both Uses of

melancholy
in
Preface to Lyrical Ballads

and, reflecting upon the magnitude of the general evil, I should be oppressed with no dishonorable melancholy, had I not a deep impression of certain inherent and indestructible qualities of the human mind, and likewise of certain powers in the great and permanent objects that act upon it which are equally inherent and indestructible;†
*

melancholy = a sad feeling or manner

On the other hand (what it must be allowed will much more frequently happen) if the Poet's words should be incommensurate with the passion, and inadequate to raise the Reader to a height of desirable excitement, then, (unless the Poet's choice of his metre has been grossly injudicious) in the feelings of pleasure which the Reader bas been accustomed to connect with metre in general, and in the feeling, whether chearful or melancholy, which he has been accustomed to connect with that particular movement of metre, there will be found something which will greatly contribute to impart passion to the words, and to effect the complex end which the Poet proposes to himself.†

Definition:

a sad feeling or manner -- sometimes thoughtfully sad