"It is said that twenty-four millions ought to prevail over two hundred thousand. True if the constitution of a kingdom be a problem of arithmetic. This sort of discourse does well enough with the lamp-post for its second;"
Just a guess, but can the "lamp-post for a second" be referring to one's conversational partner in discourse? It would seem to make sense, given the rest of the sentence.
If that's the case, the sentence means that the previously mentioned style of discourse works fine if you're talking to an inanimate object ("as dumb as a lamppost"), but it won't work if you're talking to someone with half a brain.