Ti'diixilinn SttuliiH »H 



count till! muiiiIm'I' i)t .strokes lie mi.kIc with tli»' fodt on one tiffidlc, wlnle the 

 sand-glass iiin down. This provided the nuinher of miles per hour at wiiich 

 lie was moving. For example a sand-glasfl running out in fi sees, is appro- 

 priate to a wheel of 'J fl. !>J inches dianieter, while one of 10 sees, corresponds 

 to one of 4 ft. 8 inches diameter'. I am not aware that these aand-ulaases 



with the free-wheel, several moditications would l)e nee<lful. 



ever came into use ; with the differential gearing of the modern cycle, and 



nu-gia 

 cycle. 



A last meteorological paper hy (jalton was read at the British Association 

 meeting 1880*. It is entitled: "On determining the heights and distances 

 of clouds by their reHexions in a low pond of water and in a mercurial horizon." 



"Tlie culm surfiict- of a Kheot of wiili-r, " Giiltoii writes, "may Ih> ma«lo to wive tlir purp<me of 

 a hugo mirror in a gigantic vertical range-tiiulcr, whereby a sutticieiitly largi- parallax may Ije 

 obUiiiKHl for the efVectivo measurement of clouds. The observation of the heights and thick- 

 nes.seH of the liillerent strata of clouds, and of their rates of movement, is at the present time 

 perhaps the most promising, as it is the least explored branch of meteorology. .\n then' are 

 comparatively few places in England where the two conditions are found of a pool of water well 

 screened from wind, and a station situated many feet in height above it, the author hopes by 

 the publication of this memoir to induce some qualiKe<l pi^rsons who liave acceiw to favourable 

 stations to interest themselves in the subject, and to make observations." 



The observations were to be made witli a sextant and mercurial horizon, 

 and demand a knowledge of, or a di.scovery of, the following quantities : 



(a) the difference of level between the surfaces of the mercury and of 

 the pool (</) ; 



{h) the angle between the rellection of a part (jf the cloud in the 

 mercury and in the pool (p, = « minutes of angle, say) ; 



((•) the angle between the portion of the cloud and its reflection in the 

 mercmy (2a, Galton identifies a with the altitude of the cloud and suggests 

 that it may be measured directly by a pocket altazimuth : .see our p. .^0). 



Galton gives the approximation : 



Vertical height of cloud = - 6875 "5 cos (a +/>) sin a 



and tabulates the factor by which djn must be multiplied for various values 

 of rt+|) and i>, or n, to obtain this vertical height. He mounted his horizon 

 on a bar attached to a camera tripod, so that the reflection from the pool 

 Wits seen under the mercury'. 



In this chapter of Galton's life I have endeavoured to indicate the chief 

 scope of his activities during the ten years which followed his South African 

 travels and his marriage. On his return home he came into touch with men 

 like Sir Edward Sabine and Sir Roderick Murchi.son whose etithusiastic 

 spirit caused Galton's labours to be directed in their own specialised direc- 

 tions, and the inipul.ses thus given led to phases of study the ramitications 



' Only those who remember the cycles of the 'seventies will appreciate this diameter. 

 ' Kfimrt, pp. 459-61. 



' I have checked Galton's formula of which he gives no proof on the assumption that th« 

 ot)server may Ije assumed to have his eye at the mercurj', but I have had no opportunity of 



I testing whether the method is fairly eiusy of application. Pools and clitTs are innumerable, hut 

 few of them are lutsociated. The ideal spot would hv: a disused and Hoo<ie<l quarry. 



