1) Examples in the OED date from 1516 and the meaning given is ‘a pier or supporter of a wooden bridge’: Marshall defined it in 1788 as ‘the starling of a wooden bridge’.
In some other Yorkshire examples that meaning is more or less confirmed but even in early references the bridges were sometimes made of stone: 1485-6 the making a Brygge of ston ... the which shall be made V arches embowed, iiij jowels and ij heedys with sure butments, Sheffield. In 1579, Elland Bridge was to have two substantiall juells or pyllors but again it was made of stone. In other cases the jowels were certainly of timber: 1589 Item paid for timber for making fower paire of jewells and one odd jewell, for 33 sommers and 30 planckes Ł18 19s, How Bridge. Nor is the meaning always straightforward: Wright had ‘The arch or space between the piers’ for ‘jowel’ but quoted Marshall’s definition for ‘jewel’ in a separate entry. In the 1683 lease of the Wakefield Bridge House, the jewel was more the foundation of the pier than the pier itself, for one clause in the agreement referred to any of the Stones commonly called Setters within any of the frames or Jewells under the bridg, Wakefield. The word remained in use into relatively recent times: 1745-6 Whitby Bridge, which is a county bridge, and that the jewells or supporters ... are not wide enough for the said ship to come through ... unless one inch and half of each side be chipped or cut away from the said jewells .