MERCURY IN THE ASCENDANT, OR FIRST HOUSE makes the person quick-witted and sharp according to the nature of the sign rising and the aspects of the planets which he receives. The fiery signs and the airy signs have the strongest influence in that respect.
In Aries or Leo he is more impulsive than when rising in Sagittarius, but Gemini, Libra and Aquarius are best. When Mercury is rising in the watery signs Cancer or Pisces the intellect is dull, but rising in Scorpio where it is fired by the martial energy and armed with the venomous sting of the scorpion it has a somewhat similar influence to that of Mars in Scorpio through not quite so pronounced.
Max Heindel