“And also excepting and reserving full power and liberty to the said Thomas Lord Erskine and the other heirs of tailzie to provide their younger children to reasonable provisions; declaring always, that any bonds of provision to be granted by the said Thomas Lord Erskine and the other heirs of tailzie above mentioned shall be so qualified as that any apprising or adjudication, or any other legal diligence to be led and adduced therefrom, shall only subsist for a real security for the principal sums, annual rents and expenses, but that the legal reversion of said diligences shall never expire.”