THIS IS AN UNSAVOURY subject. So, you have been warned.

On our first visit to Baroda (Vadodara) in the western Indian state of Gujarat a few years ago, I noticed several tall cast-iron structures resembling lamp posts, except that they were topped with curious objects. Each of these tall streetside poles is topped with spheres, each with four cylindrical projections. Beneath each sphere, there is an arrow. I was puzzled by these objects. So, I posted pictures on Facebook and asked if any of my friends knew what purpose these things serve or used to serve. I received several ingenious suggestions, none of which turned out to be correct.

Having seen these items in Baroda (and also in Bombay and Ahmedabad), I began noticing similar structures in London. None of the examples in London are topped with the sphere and arrow, but in other respects they are not particularly different from those I saw in India. Eventually, I learned that they are all examples of ‘stink pipes’. There are many fine specimens, disused I believe, dotted around London. Recently, I was reminded of those I saw in India when I saw a couple near Chiswick Mall in west London.

It was not until after about 1858 that sewage began being channelled through enclosed sewers in London, an improvement instigated by London Metropolitan Board of Work’s Chief Engineer Joseph Bazalgette (1819-1891). Sewage is waste matter, which tends to decompose. The products of decomposition include noxious gases such as for example hydrogen sulphide, carbon monoxide, ammonia, and methane. Some of them are highly inflammable and all of them increase in pressure as the sewage flows through the sewers. The stink pipes serve(d) as vents through which the gasses building up could escape from the sewers. They tend to be tall so that the gases blow off high above ground level where there are more likely to be breezes that can disperse the gases. Without adequate venting, the gaseous vapours can become dangerously explosive. Returning to Baroda, where the stink pipes are topped with spheres. The gases escape through the conical projections and, I have learned, the arrows indicate the direction of the sewer running beneath them and which way the sewage is flowing.