The following letter having been presented and well accepted, and some of the author's friends conceiving it may be of religious benefit, to extend it beyond its first address; : it is now reprinted for more general dispersion. A few errors are rectified which through the copier for the press mistaking the author's handwriting, or not strictly attending to the subject, were in the first publication. To the principal inhabitants of the county of Cornwall, who are about to assemble at Truro, the 29th instant, on the mining concerns of this county.