His prose style is a rare exemplification of classic severity and perspicuousness. In each paragraph the ideas arrange themselves in faultless connection, like the molecules of a crystal around its centre. The sentences are not long, the construction is simple, the words are English in its purity, without admixture of foreign phrase or idiom. But the most striking peculiarity of his diction is the utter absence of ornament; for Percival evidently held that the chief merits of composition are clearness and directness. Poetic imagery, brilliant climaxes and antitheses, fanciful or grotesque turns of expression, he rejected as unfavorable to that simple truth for which he studied and wrote. This dry, almost mathematical style, was no necessity with him; few men, surely, have had at command a richer vocabulary, English and foreign, than Percival; few could have adorned thought with more or choicer garlands from the fields of knowledge and imagination. To letter-writing he had a great aversion. I have never seen a letter or note from him to which his signature was attached. The autograph-fanciers, therefore, will find a scanty harvest when they come to forage after the name of Percival. His handwriting corresponded in some sense with his character. It was fine; the lines straight and parallel; the letters completely formed, though without fulness of curve; no flourishes, and no unnecessary prolongations of stroke, above or below the general run of the line. There were few erasures, the punctuation was perfect, and the manuscript was fit for the press as it left his hand. Literary criticism he rarely indulged in, being too disinclined to praise or blame, and too intensely devoted to the acquisition of positive knowledge. If he commented severely upon anything, it was usually the slovenly diction of some of our State Surveys, or the inaccuracies of translations from foreign languages.