The Signal ot Liberty publishes our prospectus, and wants to know by what authoFity wc say Ã¯hat the overihrow of Slavery is a Whig principie. The editor guesses we can hardly make out a case, and says that notone Whig in a hundred regards it so. Now friend, Ã¯f we are to try a case, we wi!l first look well at the issue. We did not sny it was a principie of the whole Whig party. - I3ut that it was a Whig principie. Do you take ? It is fast becoining a principie oÃ the Whig pariy in these parts, and throughout New England and New York, and wc doubt not wilt be so with the whole body of the Whig party of the Northern States before the next Presidential elction. Ãf the editor of the Signal and his friends would join in, it would help along very much in Michigan. - Ohio Truc Democrat. " The Editor of the Sjgnal and his friends " have advocated the " principie" of overthrowing Slavery for a dozen years or more, and expect to do till it shall be accomplished. This principie is a good one, and we are glud to find any Whigsdisposed to support it, having first baptized it under the name of' Wh:g principie. Your anticipations of its spread through"the whole body of the Whig party of the Northern States before the next Presidential election," we regard as rather sanguine. We suspect they would be damped somewhat by intercourse with our Michigan Whigs. Tho Whig supporters of this "Whig principie" are very scarco in this State ; and when we find one, there are ten chances to one that he isdisposed to overthrow Slavery by voting for a Whig Slaveholder for President or Vice President Ãn 1848 ! - Ask tho conductors of the Whig press what they think of your new principie. There is not a Whig paper in the State that daretnke ground openly and fairly, for the overthrow of Slavery as a Whig principie, and back it up by a refusal to vote for Whig Slaveholders. Ask your neighborof the Detroit Advertiser, and his particular friends, Senators Woodbridge, and Porter, and Mr. J. M. Howard. They would not sanction your positioneer one vioment ! These being the facts, you must see that your advice to " help along " this principie by " joining in" with the Whigs in Michigan is entirely impracticable. - We can never "join in" with Whigs, however loud in profession of antislavery principies, who will vote for slaveholders for office. When we can find those nko will talk and ac'. right on this subject, showing a right faith by good works and consistent votes, they shU have our coaperation and aid in e very proper way. The open and manly course of the True Democrat thus far, on ÃÃºs point, we are 'rank to say, meets our cordial opproiral.