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Why the Thames Froze

Between 1309 and 1814 the Thames froze hard enough to walk on at least twenty-four times. Since 1814 it has not once. The usual explanation is that the climate has warmed. That is part of it, and not most of it. Old London Bridge had nineteen arches on wide stone piers. The piers took up so much of the channel that the bridge acted as a weir, slowing the water upstream almost to a standstill. Still water freezes; moving water does not. The river was also shallower and wider then, spreading over land that has since been embanked. Shallow water loses heat faster. Old London Bridge was demolished in 1831. The embankments were built between 1862 and 1874. The river that flows through London now is narrower, deeper and faster than the one that froze — and it would not freeze in a colder winter than any of those.

How a Vaccine Teaches the Body

Your immune system learns. Meet a disease once and, if you survive it, you are usually protected afterwards. A vaccine gives the lesson without the disease. The method is old. In eighteenth-century Britain, milkmaids were known not to catch smallpox. Edward Jenner reasoned that the cowpox they caught from cattle was protecting them, and in 1796 he tested it on a boy — a procedure no ethics committee would permit now. A modern vaccine shows the immune system a harmless piece of a pathogen: a killed virus, a weakened one, or a single protein from its surface. The body makes antibodies against that piece, and keeps the recipe. Because the recipe is kept, the real pathogen is met by a defence already in place. There is a second effect. If enough people are immune, the pathogen cannot find a chain of hosts, so those who cannot be vaccinated are protected too. That effect fails below a threshold, which is why coverage rather than availability is what matters.