Domain-Specific Vocabulary

Why the Rails Are That Far Apart

Standard gauge is 4 feet 8½ inches. It is a strange number, and the reason is historical rather than engineering. Early railways were built by people who had built tramways for collieries, and those had used the wheel spacing of the carts already in use. Nobody chose it; it was inherited. Isambard Kingdom Brunel did choose. He built the Great Western at 7 feet ¼ inch, and by most measures it was better: faster, steadier, and able to carry more. He lost anyway. By the 1840s, more of the network was already standard gauge, and a train cannot run on a track it does not fit. Parliament settled it in 1846. Between 1846 and 1892 the Great Western converted its entire route. The final conversion took a single weekend and about four thousand men. The best design does not always win. The one that connects to everything else does.

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.