December 1, 1898.] 



KNOWLEDGE. 



235 



resemblance to a li,t,'litning photograph, it will be found 

 on closer examination to exhibit important differences. 

 Lightning pursues an irregular jagged course through the 

 air, but liere the curves are rounded and the band is 

 uniform in brightness from one side to the other. In the 

 so-called ribbon flashes of lightning one side of the ribbon 

 is much brighter than the other, and a series of bright 

 lines may be traced running parallel to the bright edge ; 

 for there are usually several discharges at intervals of a 

 few hundredths of a second, which, as a general rule, 

 follow the same course. The first flash, in finding a path 

 of least resistance through the air, probably jumps from one 

 dust particle to another, and having once heated the air along 

 its jagged path, the succeeding flashes, as a general rule, 

 follow the same com-se of least resistance, as if they were 

 flowing down a pipe. Ribbon flashes of lightning are usually 

 foimd to have been photographed with cameras which have 

 been held in the hand, and it may be assumed that the 

 operator has moved with the camera in the interval 

 between the flashes. But, as Mr. Symons has pointed out, 

 it is possible to conceive of a ribbon flash being produced 

 with a stationary camera. Thus, if a wind were blowing 

 at the rate of twenty miles an hour across the line of sight, 

 the moving air would, in the interval between the flashes, 

 which sometimes amounts to as much as the fifth of a 

 second, have carried the dust particles and the pathway of 

 heated air through a distance of nearly six feet. The 

 second flash, therefore, if not very distant from the fixed 

 camera, might occupy an appreciably different position on 

 the photographic plate, and a succession of such flashes 

 might give rise to a ribbon -like flash although the camera 

 was stationary. 



But in this case there is no stratification of the ribbon- 

 like band, and the band, though it differs in brightness in 

 different parts, is everywhere of the same breadth, while a 

 ribbon flash of lightning differs in breadth at different 

 parts of its path, being always broadest where the light- 

 ning path is at right angles to the direction of motion of 

 the photographic plate or of the wind, and narrowest where 

 the direction of motion is tangential to its curved path, in 

 which latter case the curve is only displaced upon itself. 



I would suggest that it is not improbable that Mr. 

 Levingston, after having made his moonlight exposure, 

 took his little camera up from the window-sill, and, 

 forgetting that the direct light of the moon would malce a 

 trace upon the plate, turned the camera upwards while he 

 looked about for the cap with which to cover the lens, or 

 while he passed the camera from one hand to the other 

 and closed the shutter over the dry plate. The block 

 (Fig. 1) is about the same size as the original negative, 

 and the white band upon it is about the twentieth of an 

 inch in diameter, which corresponds to the diameter of the 

 moon as photographed in a camera of 4i inches focus. — 



A. C. E.^XYARD.] 



OUR RAINFALL IN RECENT YEARS. 



By Alex. B. MacDowall, M.A. 



WE are all familiar with the instructive rainfall 

 map, in which the character of different 

 places, as regards wetness, is shown by 

 different shades or colours. And in the case 

 of Great Britain, the wettest places, as every- 

 one knows, are to be found among the mountains of the 

 west, while the driest are in the east. 



These maps are constructed from averages of rainfall in 

 a series of years. One naturally asks, How has the rain- 

 fall varied from year to year in different parts of these 



islands '? What is the recent history of its fluctuations ? 

 An answer to such questions is to be found in tabulated 

 figures of that excellent annual publication, Britixh 

 I'ldinfall, issued by our chief rainfall authority, Mr. G. J. 

 Symons. 



As the true meaning of a column of figures is not 

 always on the surface, I propose to call in the aid of 

 curves, with a view to better comprehension. The curves 

 here used will be smoothed curves, smoothed by means of 

 five-year averages. That is to say, each year-value will 



l-KJ 



100 



60 



20 



.jS 



5-i 



50 



46 



1840 '4 'S '52 '6 '60 '4 '8 72 '6 '80 '4 '8 



h a 



45 



43 



41 



39 



37 



50 



48 



40 



44 



Fig. 1. 



be that, not of the actual rainfall, but of an average of 

 five years. In this way the general course and signifi- 

 cance of a curve can be better appreciated. 



It might perhaps be thought that, a number of such 

 curves for different places in the country having been 

 brought together, we should find them all pretty much 

 aUke, though differing slightly in detail. It is not so. 

 While certain groups present a common type, it is possible 

 to find curves which have very little resemblance to each 

 other. 



Let us begin by considering a group of curves from the 

 west.''- Here (Fig. 1) is the smoothed curve for Keswick, 



* 111 these diagrams the riiinfall scale is varied at coQveuience. 

 The figures denote inches. Where two scales are given, the 

 respective curves are indicated by letters a and b. 



