Once again some more great photos, you certainly seemed to have the weather on your visit!
The structures that you have identified as “inspection chambers” are actually called a “Byewash” and are intended to channel a stream under the aqueduct, to prevent contamination and damage from runoff water. They are always on the upper side of the aqueduct. I’m not sure why but they seem only to be used on the first aqueduct (1855) and then only in certain locations. I think there are 6 over the full length but 3 of these are at Ballewan (Strathblane).
If you are interested most of the structures are on the RCAHMS site, if you do a search for “Loch Katrine” you’ll get 376 sites which can then be displayed on a map showing the route of the aqueducts, watch out though some are attributed to the wrong aqueduct, the link is below. You could also upload some of your own excellent photos. I’ve put together a KML file for each aqueduct which plots the route and main features on Google Earth or GPS device, there is also a KML of the RCHAMS sites which once plotted allows you to click through to the record, PM me with an email address if you want a copy.
http://canmore.rcahms.gov.uk/en/search/ ... h&show=allIt took me a while to get my head around what all the structures were actually for, some are similar in appearance but serve very different purposes, also some are not correctly classified on the RCHAMS website and on the OS maps which ads to confusion. “Pillars”, “Towers”, “Shafts” and “Manholes” are often interchanged so I put the following together to try and help.
On the 1855 aqueduct there are a number of true“shafts” these are deep vertical shafts along the tunnelled sections which were built to allow multiple work faces to be established during tunnelling and speed up the process. You often find these associated with the pyramid topped stone pillars/towers which were used for alignment purposes. Once the tunnel was complete some shafts appear to have been capped at ground level, others were left open presumably to allow access in the future if every required or perhaps to provide surge relief. The capped ones are sometimes marked with a round topped stone although over time some of these have been removed or damaged.
The uncapped ones had a circular stone tower erected to stop anything falling down, these have been covered with either a flat steel plate or a domed birdcage structure.
You occasionally come across discarded circular steel sections which must have been used to line the shafts during construction.
On the 1895 aqueduct there are similar circular stone towers with domed bridcages but these also have a gated doorway and a covered manway inside, as these structures are for access rather than for tunnelling. You find these at either end of aqueduct bridges and or at the aqueduct junction chambers which often coincide with one end of an aqueduct bridge. These are normally associated with a black steel "cheese wedge" structure for additional access, these are covers which have been added later.
On the 1855 towers my guess is that the covering was done at a later date, the flat plates seem to have been used in the north section, the birdcages in the southern section, certainly the shafts in the southern section have more attractive stone work and they may have been “tarted up” at a later date as the birdcage and stone work detail is very similar to the 1895 towers.
On the 1855 aqueduct you find a lot of square stone covered manways which are for maintenance access, here is one with a burn cover which serves a similar function as the byewash.