A translator’s prime duty is to follow his author, for which purpose he must first understand him, a requirement not very easily here fulfilled. The texts of the _Q.N._ vary greatly, as already indicated, and it is no easy matter to select any one that might be consistently followed. The most recent and best text, the Teubner, edited by Gercke, has strong claims, and had it been my good fortune to have it by me when the translation was made, I should have been tempted to adopt it _simpliciter_, even though in many details it departs somewhat violently from the accepted arrangement. As it was, it did not come to hand until the translation was finished and paged for publication, so that full use could not be made of it. In a few cases its corrections had been anticipated; in some its readings have been adopted; some that could not be incorporated are referred to in a note on the subject. The text being settled, the translator must, if possible, put himself in the author’s position and obtain his point of view. In science, particularly, the _milieu_ of the author must be caught if his thoughts are to be accurately reproduced. The danger of attributing to Seneca ideas that were unknown to him and that are due to modern analysis and discovery has to be constantly present to one’s mind. For example, “homogeneity,” “elasticity,” “electricity,” “gas,” “explosion,” etc., are a few of the terms that his language suggests, but that would probably convey a wrong impression of his conception of the phenomena to which they relate. They have been thus ruled out. Nor is Seneca consistent in the use of the terms he employs; he has no scientific vocabulary. In a separate note attention is called to his words for “air” and “atmosphere”; but there are many other terms that belong to the same category. These are, for instance, three words for “thick” or “dense,” _crassus_, _densus_, _spissus_, which he seems to use almost indifferently, at any rate without any precise discrimination. So with terms like “_impetus_” (impulse, onset) “_impulsus_” (shove, impulse), “_ictus_” (stroke, blow), “_vis_” (force, quantity, amount), “_curro_” (to run (river), to revolve (heavenly body)), and its compounds, _eo_ (to go), and its compounds, etc., etc.