Opinion by

The several assignments of error appearing on the record in this case raise but two questions, and they are questions about which the profession ought to be in no doubt. One of these is over the measure of damages to which the owner of land is entitled when his land has been entered upon under the right of *297eminent domain. The other is, what constitutes an “ expert witness ? ” The true measure of damages has been held in a long and unbroken line of cases to be the difference in the market or selling value of the property entered, before the entry was made, and afterward: Railway Company v. McCloskey, 110 Pa. 436; Railway Company v. Vance, 115 Pa. 325; Setzler v. Railroad Company, 112 Pa. 56 ; Curtin v. Nittany Valley Railroad Co., 135 Pa. 20; Mahaffy v. Beech Creek Railroad Company, 163 Pa. 158; Spring City Gas Company v. Penna. Schuylkill Valley Railroad Company, 167 Pa. 6. In this case the plaintiff was allowed to give evidence tending to show that he was the owner of a right of way appurtenant to his land that was obstructed by the building of the railroad, and that this item of damage amounted to $1,000. The evidence of the existence of the right of way was very slight and the learned judge so regarded it, as is evident from his charge. The evidence of its having any practical value to the plaintiff, if he had shown himself entitled to use it was equally slight. It is true that an “ expert witness ” had been found who placed its value at $1,000, but the learned judge took occassion to say when speaking on this subject to the jury that he would not sustain a verdict that should adopt the sum of $1,000 as the value to the plaintiff of the right of way. But he left this evidence to the jury in a manner calculated to leave an erroneous impression on their minds. He said “There is some evidence that there was a right of way there by immemorial usage, and if you come to the conclusion that there was a right of way there you may consider its value. If you come to the conclusion that there was an element of damage there you can allow it.” The jury might well understand from this instruction, indeed it is not easy to see how they could reach any other conclusion, that the right of way if found to exist might be separately valued and added to such other items as they might allow in order to make up the amount of their verdict. But the jury have no right to allow damages for distinct items, whether estimated by experts or other witnesses, and reach the amount of their verdict in that manner. Their duty is simply to ascertain the loss in the selling value of the property entered due to the fact of the taking by eminent domain. This loss stands for the measure of damages because it embraces the effect of all the elements of depreciation taken *298together. The other question ought to be equally free from difficulty. An expert is a person experienced, trained, skilled in some particular business or subject. An expert witness is one who because of the possession of knowledge not within ordinary reach is specially qualified to speak upon the subject to which his attention is called. Thus, a chemist, a physician, a mechanic, an artist, has special knowledge of the things that fall within the range of his studies and his daily practice,' and because of such special knowledge, not within ordinary reach, his testimony upon a subject relating to his particular line of study or research is regarded as more exact and entitled to more weight than that of witnesses not possessing the same opportunities for acquiring thorough knowledge of the subject. Many persons may know something about a given question and be competent as witnesses to tell what they know. A few may have an intimate, an exceptional, knowledge and be entitled to speak as expert witnesses. Now the question to which the so-called expert witnesses were called in this case was the value of the plaintiff’s land before the location of the defendant’s railroad over it, and its value as affected by that location. That-they were experts in the value of real estate elsewhere did not give to their testimony the value of expert evidence when they spoke of the plaintiff’s property. If they had no knowledge of the prices of land in that neighborhood before and after the location of the railroad, they ought not to be allowed to guess from their knowledge of prices in some other neighborhood, and have such guess left for the consideration of the jury as expert testimony. Sprogle was a witness, but he certainly did not show himself to have knowledge not within ordinary reach on the subject of the value of the plaintiff’s land. He had no special or peculiar knowledge of values in that neighborhood. He knew enough about the subject to be entitled to be heard as any other witness might be, but expert knowledge means more than that. It was a mistake therefore to treat or submit to the jury his evidence as that of an expert in values in the vicinity in which the plaintiff’s land was situated. His evidence should have gone with that of other witnesses who spoke as witnesses simply; or in the language of the learned judge found in the bill of exceptions, “I think he can testify as far as his testimony goes.” He might have been an expert in values in Philadel*299phia or Reading, but he did not show himself to be such as to Delaware county. The knowledge relied on to give the testimony of a witness the value of that of an expert must relate to the subject under investigation: Spring City Gas Works v. Penna. Schuylkill Valley Railroad Company, 167 Pa. 6. The judgment is reversed and a venire facias de novo awarded.