Mr. FRANCIS GALTON said it might seem strange that there should be an error of 1000 feet of altitude suspected in the observations of an explorer. Here, in England, levels were made to an inch, but the method of operating in uncivilised countries was quite different from that employed at home. Instead of using a spirit-level, and taking sights, the traveller in Africa had to boil a thermometer, that is, plunge it into boiling water in order to see at what temperature the water was. At the top of a high mountain, such as Mont Blanc, the temperature of boiling water would be insufficient to boil a potato. One degree of temperature corresponds to 500 feet in height ; two degrees of temperature would be 1000 feet. The thermometer that Speke used, his others having been broken, was neither more nor less than a bath thermometer, and could not be depended on to a couple of degrees, especially after it had been taken a long journey in Africa and exposed to a dry climate. Even the glass of the best thermometers, after being often put into boiling water, becomes altered in structure, and the instruments change their boilingpoint. Consequently, with an untested wooden thermometer, such an error as Speke bad made was a very small matter.
Discussion on Dr. Livingstone's Letters
Proceedings of the Royal Geographical Society