LECTURE BY "MARK TWAIN."--In reply to a letter signed by many prominent citizens, all the military, "various benevolent societies, citizens on foot and horseback, and 1,500 in the steerage," beseeching Mark Twain to desist in his proposed lecture to-morrow night, at Mercantile Library Hall, he published the following:

  To the 1,500 and Others: It seems to me that your course is entirely unprecedented. Heretofore, when lecturers, singers, actors and other frauds have said they were about to leave town, you have always been the very first people to come out in a card beseeching them to hold on for just one night more, and inflict just one more performance on the public--but as soon as I want to take a farewell benefit you come after me with a card signed by the whole community and the Board of Aldermen, praying me not to do it. But it isn't of any use. You cannot move me from my fell purpose. I will torment the people if I want to. I have a better right to do it than these strange lecturers and orators that come here from abroad. It only costs the public a dollar apiece, and, if they can't stand it, what do they stay here for? Am I to go away and let them have peace and quiet for a year and a half, and then come back and only lecture them twice? What do you take me for?

No, gentlemen, ask of me anything else and I will do it cheerfully; but do not ask me not to afflict the people. I wish to tell them all I know about VENICE. I wish to tell them about the City of the Sea--that most venerable, most brilliant, and proudest Republic the world has ever seen. I wish to hint of what it achieved in twelve hundred years, and what it lost in two hundred. I wish to furnish a deal of pleasant information, somewhat highly spiced, but still palatable, digestible, and eminently fitted for the intellectual stomach. My first lecture was not as fine as I thought it was, but I have submitted this discourse to several able critics, and they have pronounced it good. Now, therefore, why should I withhold it?

Let me talk this just this once, and I will sail positively on the 4th July, and stay away until I return from China--two years.

The box plan will be exhibited this morning, and in the language of the lecturer, "the orgies will commence at 8" to-morrow night.

"MARK TWAIN'S" LECTURE TO-NIGHT.--Seats for the lecture to-night at the Mercantile Library Hall were so rapidly taken while the box office was open yesterday, that the lecturer instructed his agent to withdraw the announcement "children under one year, half price," as it was evident the space would all be required for grown persons. Although "Mark" intends to give a good deal of information with his humor, to-night, he wishes the public to understand that he will not detain them more than two hours with his speech, no matter how much he may be applauded. "The Oldest of the Republics, Venice, Past and Present," is the title of the lecture.