That we were gradually overhauling the chase was evident from the fact that we were slowly raising her, while she was unable to head-reach upon us; and at sunset we could see the foot of her topsail from the deck while she had not altered her bearing from us by so much as a quarter point since we had last hauled our wind.  And if we in the _Francesca_ were gaining upon her, the _Dona Inez_ was doing so in a still more marked degree, that craft being, at the time last-mentioned, quite eight miles ahead of us, and about two points on our weather bow.  The question now arose in my mind whether she would endeavour to dodge us during the night?  She would find it exceedingly difficult to do so, for there was now a good moon in the sky, affording sufficient light to enable a man with keen eyes to keep a craft at her distance from us in sight without very much trouble; but, on the other hand, there was a very heavy mass of cloud banking up to windward and fast overspreading the sky.  This would obscure the moon later, and perhaps for a time cut off enough of her light to give the stranger a chance, should he wish to avail himself of it.  I therefore sent one of the keenest-sighted men I had with me up on the topsail-yard as soon as it began to grow dusk, with instructions to keep his eye on the stranger and immediately report to me should he happen to lose sight of her.  For we knew, both from hearsay and experience, that the slavers were as wily as foxes, and were in the habit of adopting all sorts of queer expedients to evade pursuit. Not content, therefore, with sending a hand aloft to watch the stranger, I maintained an almost continuous watch upon her myself from the deck with the aid of the _Francesca's_ excellent telescope, which was both a day and a night glass.