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The Union Canal is one of two Lowland canals in Scotland. After four years' construction it opened in 1822 and was known as the Edinburgh and Glasgow Union Canal. The name reflected the role of the canal, linking Edinburgh with the Forth and Clyde Canal at Falkirk and so providing a through route between Scotland's two major cities.
There are three major aqueducts on the Union Canal. The 500ft long eight-arched Slateford Aqueduct on the edge of Edinburgh takes the canal over the Water of Leith, while another takes the canal over the River Avon west of Linlithgow. This is 810ft long and 86ft high, making it the second longest in the UK. Arguably, however, the most spectacular of the three aqueducts is also the smallest. The Lin's mill Aqueduct west of Ratho carries the canal over the River Almond in five arches, 75ft above the river, with much of the character of the crossing coming from the steep sided valley of the river, especially on its eastern shore.
There is a canal basin and wharf at the east end of the Lin's Mill Aqueduct. Nearby is a car park reached by a track from a corner on a minor road a little south east of the Clifton Hall School, and this is the best place from which to access the aqueduct. From the car park you can stroll across the aqueduct on its southern side, enjoying the views down into the valley of the River Almond below. A milestone at the western end of the viaduct indicates it is 10½ miles to Edinburgh and 21 miles to Falkirk.
The more adventurous can use the steps beside the east end of the aqueduct to descend to a track leading to Lin's Mill, pass under the canal, and reclimb more steps to reach the towpath on the north side of the canal. Gaining the side of the River Almond, 75ft below the level of the aqueduct, requires some slight scrambling. The effort is worth it, because it is only from the side of the river that you can really begin to appreciate the bulk and dramatic setting of the aqueduct as it towers above you.
Returning to the car park it is worth taking a look at what at first appears to be a side arm of the canal heading north from the eastern end of the aqueduct into a low, stone-lined tunnel. This is the Lin's Mill Feeder. It is three miles long and helps provide a link which allows water from the Cobbinshaw Reservoir, on the northern side of the Pentland Hills, to keep the canal topped up. An overflow on the Lin's Mill Aqueduct ensures that the canal never becomes too full: any excess water is simply allowed to pour into the river below.
The Lin's Mill Aqueduct was designed by Hugh Baird. The original idea was to build an ambitious structure comprising just a single arch. Baird sought the advice of the acknowledged master of aqueduct building at the time, Thomas Telford, on his design. The result was a radical change of plan, and what emerged was closely modelled on the aqueduct Telford had built 20 years earlier at Chirk on the Ellesmere Canal.
Construction began in 1819, but was soon halted because of problems caused by faulty materials and poor workmanship. The contractors were replaced and the work restarted. Like the rest of the canal, the aqueduct was completed in 1822.