The market value of land even in cities is not capable of exact proof, like that of stock or the staple commodities of trade, such as flour, or sugar, or pig iron. At best it is matter of opinion not strictly of experts, but of those who are shown to be familiar with the neighborhood, the situation and availability of the particular tract, the buying and selling value as shown by such sales or transactions as have occurred under circumstances to make them fair bases for inference as to the particular matter in issue. In neighborhoods where sales are few and at long intervals it would be unfair and impracticable to require as full and detailed knowledge on the part of witnesses as in other localities where sales are frequent and of public interest and attention. Opinion of any kind is a poor quality of *438evidence, and where admissible at all is only so because it is the best that is available. In all questions of competency on that subject there must be a sliding scale, the only standard of which is that the witness shall have such knowledge of the subject-matter as can be reasonably expected in view of the circumstances of the particular case. The witnesses in the present case appear to have come fairly up to that requirement: Leiby v. Clear Spring Water Co., 205 Pa. 634 (644) ; Smith v. Penna. R. R. Co., 205 Pa. 645; Hope v. Phila., etc., R. R. Co., 211 Pa. 401.

Judgment affirmed.