344 



NATURE 



[November ii, 1920 



one second (000028 cm.) is spread while falling over 

 a vertical column of air 3000 cm. high. 



There is, therefore, i c.c. of water in about 10' c.c. 

 of air, or I raindrop in every 5 litres or so . . . . (i) 



Now, in the state of maximum disorder (the "most 

 probable" state) the raindrops will be spaced equally 

 in all directions, vertically as well as horizontally, 

 and the average distance between two spots will be 



•^5000=17 cm (2) 



^ jc H-+ 



Consider a circle of landscape, of diameter AB, 

 subtending an angle d at the observer at O, distant 

 I from AB. AB = Z.e. 



A spot of rain at x is projected against the back- 

 ground as a disc of diameter 

 l.d 



X 



(d being the diameter of the spot) and blots out an 

 area 



If n denotes the number of rain spots in unit volume 

 of air, then in the element dx of the cone in the 

 figure there are 



ft . -(.r . ey-dx, 



4 

 and these, projected on the background, blot out an 

 area 



or 



\4 -^" / 



4 4 



or 



«- . d'^ . dx X Area of background . (3) 

 4 

 Hence each element dx contributes the same 

 amount of blotting-out of the landscape, the larger 

 apparent diameter of the nearer spots being com- 

 pensated by their smaller numbers, and the visibility 

 of the landscape falls off as a linear function of the 

 distance. 



The total area blotted out is the integral of (3), 

 ■• Area of projected rain spots _ //t jjX 



Area of landscape \4 /' 



and the landscape is entirely blotted out when the 

 above ratio equals unitv. 

 Putting 



_ Depth of rain falling per sec. (D) 



. . d^ . V 

 6 , 



in this equation, we fjet as the limiting distance at 

 which the outlines of the landscape can be distin- 

 guished 



« d . V , , 



"d" ^4) 



/=: 



Using the values suggested above, d = oi, ^ = 3000, 

 and D = 28xio-°, the value of \ comes out as 7 km., 

 or 4 J miles. 



Thus a very heavv rainstorm may be remarkablv 

 transparent. 



Let N = the number of spots falling per second on 



NO. 2663, VOL. 106] 



unit area (N = n.r), then, combining equation (4) with 

 Stokes's law, and calling - the obscuring power of 

 the storm, we have 



'^=9T_^ N (sincep-ft=i) . . (5) 



That is, the obscuring power of a storm is simply 

 proportional to the number of spots falling per second, 

 or every raindrop has the same obscuring power, 

 whether it be large or smalt and whether the total 

 rainfall be heavy or light. 



The very great obscuring power of a " Scotch 

 mist " is thereby accounted for. 



F. \V. Preston. 



90 Howard Road, Leicester, October 21. 



Museums in Education. 



In Nature of October 28 appeared a review of the 

 Final Report of the Committee (British Association, 

 Section L) on " Museums in. Relation to Education," 

 in which it was stated that " our great public schools 

 have some excellent museums, but there is little or 

 no evidence that they are used in school teaching." 

 May I be allowed to point out that this sweeping 

 indictment of incapacity or lack of imagination on 

 the part of masters in public schools is not without 

 exception ? 



The museum of natural history at Oundle School 

 is, I suppose, fairly typical of such collections. It 

 consists of specimens representative of zoology, 

 botany, pateontology, and petrology. Owing to lack 

 of space, not all the specimens can be exhibited with 

 advantage, and the excess has been temporarily 

 stored, so as to avoid detracting from the educational 

 value of the exhibits, pending the erection of a more 

 spacious building. The present museum, which is 

 apart from the laboratories, is accessible at all times 

 to all bovs whether or not they are taking natural 

 history subjects in the curriculum. The zoological 

 collection forms an index to the animal kingdom ; it 

 consists of specimens representative of all the phyla 

 and of a considerable number of classes. These are 

 distinctly grouped and clearly labelled. In further 

 amplification of this series a certain number of 

 drawings, dissections, and skeletons, also clearly 

 labelled, are interspersed among the types. This work 

 has been carried out entirely by the boys, under 

 supervision, and forms, therefore, an elementary 

 introduction to research. There are also table-cases 

 representative of protective coloration, insect pests, 

 metamorphosis, etc. Owing to lack of time and space 

 the botanical exhibition is not so far advanced, but 

 there is' a good collection of types of timber on view, 

 while a collection representative of the dispersal of fruits 

 and seeds is in the making. During the summer term 

 the younger bovs maintain a constant display of the 

 flowering plants of the neighbourhood. (The lack of 

 botanical exhibits in the museum is largely compensated 

 for bv a botanical garden comprising natural-order 

 beds, a rock garden, a rose garden, marsh and aquatic 

 flora, etc.) The oalffiontological collection consists of 

 a series of w-all-cases containing fossils arranged 

 according to their strata. The petrological collection 

 comprises table-cases illustrative of the more common 

 rock-forming minerals, with a considerable number 

 of good specimens clearlv arranged and classified. 

 For the two latter collections, as regards both 

 material and arrangement, the school is indebted to 

 the great kindness of Profs. Marr and Solly respec- 

 tively. 



More fhan three hundred bovs (out of five hundred) 

 are undergoing biological training at Oundle School 



