54 Life and petters of Francis Galton
ocean being divided into areas of 5° angle in longitude and latitude, and the ship returning all its observations, subject to the sole condition of an interval of eight hours between observation and observation, a ship will give more observations when the wind is unfavourable than when it is favourable ; accordingly there will be an error produced-since favourable and unfavourable winds are peculiar to certain areas, and ships outward and inward bound follow different courses-in taking not only the mean direction of the wind for certain areas but also in other meteorological variates highly correlated with the wind, such as temperature and dampness. The remedy would be to enforce not only an interval in time, but an interval in distance of the positions of successive observations.
Galton's criticism is of less importance now that steamships have replaced sailing vessels, but the paper is of interest as marking probably the first occasion on which Galton exhibited publicly his fine instinct for the discovery of statistical fallacies.
The reader will not appreciate Galton's work at this period unless he remembers that Galton's earliest travels were associated with sailing ships; it was, in such a vessel, the Dalhousie, that he sailed for Africa; and he thougt for many years of his life in terms of wind and not steam as a motive power'. Thus it came about that when Galton turned his study of meteorology in the direction of ocean travel, he thought in terms of sailing vessels. The wind had for Galton a singular fascination, and for him the problem always was: What can we learn from the wind, how can we make it of greatest service?
Three or four of his papers touch on wind problems, and these we will now briefly consider.
The first one that may be referred to is entitled: "Barometric Predictions of Weather," and the paper was read at the British Association Meeting in 18702. Galton's paper is suggestive, because, what he is actually seeking for in his linear prediction formula of the velocity of the wind in terms of -barometric height, temperature and damp is what is now familiar to statisticians as a multiple regression formula. Galton very properly saw that the relation of barometric height to wind-velocity did not depend upon the instantaneous wind, and he accordingly experimented with average windvelocity for a series of two, three, etc. hours. He came to the conclusion that -the best period for the average was about twelve hours. He considered that twelve hour averages should also be taken for temperature and damp. Galton easily found his averages from the automatic record of continuous temperature, wind-velocity and damp. He explains clearly why he takes an average, namely the barometric pressure acts in sympathy with a much larger wind-velocity area, than that immediately in its own neighbourhood. The pressure (as in the case of water) is affected some time
' I think this is true even as late as the early 'seventies when Galton was busy with his "wave engine" (see p. 51). Such an engine as a propulsor would hardly have occurred to one who had grown up in an era of steam vessels.
2 Brit. Assoc. Report, 1870, Trans. Sections, pp. 31-33; Nature, Vol. ii, Oct. 20, 1870, pp. 501-3.