2012, Andrew Martin, Underground Overground: A passenger's history of the Tube, Profile Books, →ISBN, page 61:
Investors were now warier of Underground railways. They had seen that the Metropolitan was bringing in lower returns, having broached its City extensions. But the newly established Metropolitan Board of Works (the first London-wide authority) pressed the District to continue with the next part of the plan - Westminster to Blackfriars - so that the building of the railway could go in tandem with the construction of the new Thames Embankment and the sewer it incorporated.