Four steps to Longitude! If we chose, we might interpret the phrase by a sequence of four names: Ptolemy, Charles II, Tobias Mayer and Nevil Maskelyne—the last in 1763. But in fact the steps represent the four marine timepieces made by John Harrison—familiarly known as H1, H2, H3 and H4 of which the last proved its efficacy in 1762. The longitude was found, and by a ‘machine’, just two centuries ago. Now the bi-centenary is being celebrated by a very elegant and informative exhibition at the National Maritime Museum, Greenwich. No navigator should miss it. Even if he thinks scorn of ‘ancient history’ he owes the inventor of the first English chronometer a tribute of respect.