Mere abstract opinion is not evidence; but a surveyor, or any other person conversant in the subject, may state facts, and his opinion on those facts, to enable the jury to form a correct judgment of the matter in dispute. It is general information in a question of science, which others unacquainted with the subject must necessarily want. Thus a physician, who has not seen the particular patient, may, after hearing the evidence of others, be called to prove on his oath, the general effects of a particular disease, and its probable consequences in the particular case. Peake on Evid. 137.