The memoranda by Jones relative to these defences (printed for private circulation) form a most valuable military work, fully describing the various field-works forming the lines of Torres Vedras. All the arrangements for manning the works and placing the troops had been so well made by Jones that the several points were occupied as quickly and with as much regularity as if the troops had been re-entering their cantonments from a review.
On 17 Nov. 1810 Jones was appointed brigade-major of engineers in the Peninsula, and was attached to the headquarters' staff, the details of the engineers' service in all parts of the Peninsula passing through his hands.
Jones held the appointment until May 1812, and was employed at all the sieges undertaken during that period. For his conduct during the operations against Ciudad Rodrigo he was particularly mentioned by Wellington in his despatches, and in consequence was gazetted brevet-major on 6 Feb. 1812. At the siege of Badajoz Fletcher, the commanding engineer, was wounded, but at the express wish of Wellington retained his command, and the active duties therefore devolved upon Jones, his staff officer. In the assault of Fort Picuriaz Jones saved the life of Captain Holloway of the engineers, who had been shot down on the parapet and fell on to the fraise. For his exertions at the siege Jones was gazetted on 27 April 1812 brevet lieutenant-colonel, and he thereupon resigned his appointment as brigade-major.
When it was determined to carry on operations on the eastern coast of Spain, Jones was appointed commanding engineer under General Maitland, and sailed from Lisbon in the beginning of June. On the disembarkation of the troops at Alicante, Jones received an appointment on the staff as assistant quartermaster-general, there being already an engineer officer senior to himself in command of the engineers. Owing to differences between the commanders of the allied forces, Jones was sent on a special mission to Madrid, to explain to Wellington the position of affairs. Travelling by night and avoiding roads, Jones reached Madrid safely, and was warmly received by Wellington, who, sending instructions by a courier, kept Jones to accompany him to the north to the siege of Burgos. During the progress of that siege, Jones was instructed to signal to Wellington by holding up his hat when the arrangements for exploding a mine and making a lodgment were complete. As the signal was not acknowledged, Jones repeated it until the French perceived him, and their fire brought him down with a bullet through his ankle. He with difficulty rolled himself into the parallel, but he ordered the mine to be fired, and the operations entrusted to him were successfully carried out before he left the field. Jones remained in a state of delirium for ten days, and as soon as he could be moved Wellington sent him to Lisbon in the only spring wagon at headquarters. The sufferings of this two months' journey severely tried his strength, and he remained in Lisbon until April 1813, when he was sent to England. Eighteen months of severe suffering followed. During this period he composed and published a volume entitled ‘Journal of Sieges carried on by the Allies in Spain in 1810, 1811, and 1812.’ In this work he fearlessly exposed the deficiencies of the engineer service, which he attributed to the ignorance and military incapacity of the board of ordnance. These strictures naturally offended the dispensers of patronage. Wellington, however, although the book was published without his sanction, and sharply criticised his siege proceedings, praised it, and remained the author's friend.
In 1814 Jones visited the Netherlands, examined the principal fortresses, and afterwards met Wellington at Paris. Wellington told him that he had appointed him, with Brigadier-general (afterwards Sir) Alexander Bryce [q. v.] and another engineer officer, to report on the system of defence for the new kingdom of the Netherlands. The commissioners arrived in Brussels 21 March 1815. On 4 June 1815 Jones was made a C.B. On the appointment of Wellington to the command in the Netherlands, Jones accompanied him round some of the principal points of defence. At the end of August the reports of the commission were taken to Paris by Bryce and Jones and submitted to Wellington, with whom all details were settled by March 1816, when the commission was broken up. Jones was then selected to be Wellington's medium of communication with the Netherlands government for the furtherance of the objects of the report. In the previous December Jones, with Colonel Williamson of the artillery, acting as commissioners of the allied sovereigns, prevented the fortress of Charlemont from falling into the hands of the Prussians. The commissioners then took possession of Landrecy for the allies, and returned to Paris in January 1816.
In November 1816 a convention founded on the treaty of Paris was signed between England and Holland, empowering Wellington to dispose of a fund of six millions and a half in constructing defensive works for the protection of the Netherlands, and to delegate