Armstrong was a man much beloved and respected by his intimates, and seems to have possessed great goodness of heart, as well as extensive knowledge and abilities; but a kind of morbid sensibility preyed on his temper, and a languid listlessness damped his intellectual efforts. The following lines in Thomson's Castle of Indolence are said to have been meant for his portraiture:—
With him was sometimes join'd, in silent walk,
(Profoundly silent — for they never spoke,)
One shyer still, who quite detested talk;
Oft stung by spleen, at once away he broke,
To groves of pine, and broad o'ershadowing oak,
There, inly thrill'd, he wander'd all alone,
And on himself his pensive fury wroke:
He never utter'd word, save when first shone
The glittering star of eve — "Thank heav'n! the day is done."
It should not be forgotten, that Armstrong contributed to this excellent poem the fine stanzas descriptive of the diseases to which the votaries of indolence finally become martyrs.
His reputation as a poet, is almost solely founded on his Art of Preserving Health, for his other pieces scarcely rise above mediocrity. This may well rank among the first didactic poems in the English language; but though that class of poetry is not of the highest order, yet, the variety incident to his subject has given him the opportunity of displaying his powers on some of the most elevated and interesting topics, and they are found fully adequate to the occasion. The work is adopted into the body of English classics, and has often been printed, both separately, and in collections. The following character of Armstrong's style and manner, is given in an essay prefixed to an ornamented edition of the poem, printed for Cadell and Davies, 1795. "It is distinguished by its simplicity — by a free use of words which owe their strength to their plainness — the the rejection of ambitious ornaments, and a neat approach to common phraseology. He sentences are generally short and easy; his sense clear and obvious. The full extent of his conceptions is taken in at the first glance; and there are no lofty mysteries to be unravelled by a repeated perusal. What keeps his language from being prosaic, is the vigour of his sentiments. He thinks boldly, feels strongly, and therefore expresses himself poetically. Where the subject sinks, his style sinks with it; but he has for the most part, excluded topics incapable either of vivid description, or of the oratory of sentiment. He had from nature a musical ear, whence his lines are scarcely ever harsh, though apparently without much study to render them smooth. On the whole, it may not be too much to assert, that no writer in blank verse can be found more free from stiffness and affectation; more energetic without harshness, and more dignified without formality."