Clark returned to consultancy, and patented a device for preventing smoke from being emitted when coal was burned in locomotive fireboxes. The primary feature of this device was a series of air inlets in the firebox sides, air being forced in by steam jets when the regulator was closed. It was invented in 1857, tried out by the North London Railway and the Eastern Counties Railway, and following further trials in 1858 by Clark's former employer, the GNoSR, was adopted in 1859 as a standard fitting by the latter railway. It was used on all GNoSR locomotives built during the terms of office of Clark's successors Ruthven (1855–1857) and William Cowan (1857–1883), and was still being fitted to new engines as late as 1890 by Cowan's successor James Manson, although in modified form.
