King James the First considered his lordship no longer as a tenant, but gave Sion to him and his heirs for ever. Many improvements were made in his time; for it appears from one of his lordship’s letters to the King in 1613, that he had laid out 9000_l._ in the house and gardens; which sum was probably expended in finishing them according to the Protector’s plan. His son Algernon, afterwards appointed Lord high Admiral of England, succeeded to the estate in November 1632. He employed Inigo Jones to new face the inner court, to make many alterations in the apartments, and to finish the great hall in the manner in which it at present appears.
It must not be omitted in the history of this place, that the Dukes of York, and Gloucester, and the Princess Elizabeth were sent hither by an order of the parliament agreed upon August 27, 1646, and according to Lord Clarendon were treated by the Earl and Countess of Northumberland in all respects as was most suitable to their birth. The unhappy King frequently visited them at Sion in 1647, and thought it a very great alleviation of his misfortunes to find his children so happy in their confinement. The Duke of Gloucester and the Princess Elizabeth continued at Sion till 1649, at which time the Earl resigned them to the care of his sister the Countess of Leicester.
May 30th 1682, Charles Duke of Somerset married the Lady Elizabeth Percy, the only daughter and heiress of Josceline Earl of Northumberland, by which means Sion and the immense estate of the Percies became his Grace’s property. The Duke and Duchess lent this house at Sion to the Princess of Denmark, who honoured it with her residence during the time of a misunderstanding which arose between her royal highness and her sister Queen Mary.
Upon the death of Charles Duke of Somerset, December 2, 1748. Algernon Earl of Hertford, his only surviving son succeeded to the title and a vast estate, and soon after gave Sion to his daughter and son-in-law, the present Countess and Earl of Northumberland, to whose fine taste and liberality are owing the many and great improvements which have made the gardens at Sion so universally admired.
The old Gardens, as we have already observed, were indeed very grand and magnificent according to the fashion of the age in which they were made, but, in consequence of the taste that then prevailed, they deprived the lower apartments of almost every advantage of prospect, which the fine situation of Sion house naturally affords. To make the necessary alterations required nothing less than his Lordship’s generosity. Accordingly the high triangular terrace, which the Protector had raised at a great expence, was removed, the walls of the old gardens were taken down, and the ground before the house levelled, and it now forms a fine lawn extending from Isleworth to Brentford. By these means also a beautiful prospect is opened into the King’s gardens at Richmond, as well as up and down the Thames. Towards the Thames the lawn is bounded by an ha-ha, and a meadow; which his lordship ordered to be cut down into a gentle slope, so that the surface of the water may now be seen even from the lowest apartments and the gardens. In consequence of these improvements, the most beautiful piece of scenery imaginable is formed before two of the principal fronts, for even the Thames itself seems to belong to the gardens, and the different sorts of vessels, which successively sail as it were through them, appear to be the property of their noble proprietor.