THE EIGHTEENTH GREAT FAIR OF THE AMERICAN INSTITUTE at NIBLo's GARDEN, New-York,-The Fair will open on Monday, the 6th of October, next, at 12 o'clock, M. The days for delivering, articles at the Garden, THURSDAY, Friday and Saturday previous. The National Convention of Farmers Gardeners, and Silk Culturist, I will be held on Thursday, 9th October. Delegates, without, further notice are invited from all parts of the Union. The Cattle Show will be held on the ground between 23d and' 24th streets, 5th Avenue. Ploughing 'and Spading Matches & C., as usual. Head Quarters for Managers, Committees & c., secured in the Madison Cottage. Fine horses for draft, healthy fat cattle, and sheep for market, well matched and trained working cattle, and new and useful inventions, will command higher premiums than at any preceding Fair. An opening, anniversary,, & C. Address will be delivered by the most eminent orators of our country. The people of this great Republic are invoked zealously to co-operate in carrying out this exhibition, which promises far to exceed anything that has gone before. au23