Professor H. M. Stephens, of the University of California, secretary of the Academy of Pacific Coast History, will deliver the third of his course of four Lowell Institute lectures on "The San Francisco Earthquake" in Huntington Hall, 491 Boylston street, Boston, this evening at 8 o'clock. The topic for today's lecture will be "The City of Ruins." The final lecture, "The Relief of a Ruined City," will be given Friday. Admission to the lectures is free.