In estimating the compensation to be given for any such last-mentioned lands, or any estate or interest in the same, or for any mesne profits thereof, the jury, or arbitrators, or justices, as the case may be, shall assess the same according to what they shall find to have been the value of such lands, estate or interest, and profits, at the time such lands were entered upon by the promoters of the undertaking, and without regard to any improvements or works made in the said lands by the promoters of the undertaking; and as though the works had not been constructed.