182 



FOREST INFLUENCES. 



place two or more similar qaufies near it, and similarly located as far as obstacles 

 are concerned, except only that one of these is to be decidedly lower than the old 

 one and the other decidedly higher. From a comparison of the simnltaueous rec- 

 ords of any two gauges and their altitudes, we should for each separate rainfall, 

 rather than for the monthly and annual sums, deduce the normal rainfall by the so- 

 lution of two or more equations of the form: 



Observed catch of gauge = (1 — x "/altitude) X (desired catch of normal pit gauge). 

 Where x is the unknown special coefficient of deficiency due to wind at that al- 

 titude — that is to say, having two gauge catches, ci and Cjfor the two altitudes Hi 

 and flj— we obtain the true rainfall {R) by the formulai: 



c, = (l — x^/5^) jj 

 « Ci={l—xVH.i)R 



whence 





VUz — VH^ 



^k 



(Ci— C )=C|-f H {Cx—Ci). 



If Ci and H] relate to the lower gauge we shall generally have Ci > Cx and fl^i <^ Hi 

 and the coefficient n will be a positive fraction, whose value is given in the follow- 

 ing table for such combinations of units as may easily occur in practice. 



Table VI. 



If the present gauge is located upon the top of a building, perhaps the best that 

 can be done to study the accuracy of its records is to locate other similar gauges so 

 as to get the average rainfall over the whole roof at the same uniform altitude ; the 

 next best would be to establish a standard protected or shielded gauge as high as 

 practicable above the roof. 



If a new observing station is to be started then a single shielded or protected 

 gauge isbetterthana single unprotected one; buttwo more shielded gauges at differ- 

 ent altitudes afford the means of calculating the correction for wind which will, of 

 course, be quite small for this style of gauge. 



VARIATIONS IN GEOGRAPHICAL DISTRIBUTION OF RAINFALL, 



By the combination of records from widely separated rainfall stations we ordina- 

 rily seek to determine the uniformity or irregularity of rainfall as to its geographi- 

 cal distribution. The study of horizontal distribution of rain should be first made 

 by means of simultaneous observations at many stations within a small region. The 

 most instructive work of this kind that I know of is that just now being carried on 

 by Hellmann in the " experimental rainfall field" of the Royal Prussian Meteorolog- 

 ical Institution. This institution was iu 1884 officially transferred from the bureau 

 of statistics, where it bad been organized by Mahlmann and Dove, over to the bu- 

 reau of religion, education, and medicine, where it is now intimately connected 

 with all the scientific and educational work in Berlin and is under the directorship 



