The new scheme worked well until 1842, when there was a severe drought, resulting in many of the smaller streams and brooks drying up completely. Less than of rain fell between April and October, and the Glencorse Reservoir emptied. Millers on the River Esk demanded that water from the Crawley springs should be diverted into the Glencorse Burn, and this was only avoided by paying them an indemnity of between £4,000 and £5,000. The Company rushed through an Act of Parliament in 1843 to allow them to abstract water from streams at Listonshiels, Bavelaw and Black Springs, all of which are on the north side of the Pentland Hills. Two more reservoirs were required to provide compensation water, at Threipmuir and Harlaw, and despite difficulties in buying the required land, Jardine pressed on with the project. However, he retired in 1846 before either reservoir had been finished, having reached the age of 70, and it was James Leslie who took over and saw them completed. Leslie had previously worked with Jardine on a report about a water supply scheme for Dundee, for which an Act of Parliament was obtained in 1837, but no work was undertaken at the time. Construction of the dam for the Threipmuir Reservoir may have been completed before Jardine retired, but the Harlaw dam was definitely Leslie's work. The main outlet for the reservoir was through a cast iron pipe sitting on the foundations for the dam. A masonry tower was built at the inner end of the pipeline, near the upstream toe of the dam, and a rod passing up the tower enabled the sluice to be controlled. To enable the outlet valve to be inspected or maintained, a sluice made of oak could be lowered in guides, cutting off the incoming water and allowing the pipeline to drain. A second similar structure allowed water to be taken from about above the bottom of the reservoir. When the dam was nearly completed, there was evidence of slippage on the upsteam face, which displaced the tower slightly. Leslie put this down to the dam being built too quickly, with insufficient attention being paid to consolidating the hard boulder clay from which much of the dam was constructed, and all his subsequent dams were constructed in layers less than thick, to mitigate such problems. Despite the movement of the tower, no remedial work was undertaken, and it has remained standing ever since. Leslie completed the work speicified by the 1843 Act in 1848.