180 



FOREST INFLUENCES. 



For gauges higher than 6 meters this table gives the results of the individual lo- 

 calities. If we consider the individual ligures in the latter part of this table it 

 would seem that the diminution of rainfall with elevation of gauge is decided, but 

 irregular; but it is proper to collect the data into a few mean values as shown in 

 the following table, in which the three higher groups may be considered to repre- 

 sent the average conditions of the precipitation in the temperate zone q^uite as 

 fairly as do those of the lower altitudes: 



Table V. 



If we may assume that on the average of the years and of the localities thus 

 grouped together there is a fairly uniform average quality of precipitation, we 

 should expect the deficiency at each altitude to have some definite relation to the 

 velocity of the wind, and it emphasizes our conviction that the wind is the principal 

 factor in bringing about this deficit when we find that these normal percentages are 

 fairly represented by the simple formula : Deficit=6 per cent of the square root of 

 the altitude expressed in meters. The adoption of the simple square root of the alti- 

 tude is of course suggested by the well-known studies of Stevenson and Archibald, 

 from which I infer that for these low altitudes the square root is a satisfactory ap- 

 proximation to the rate of increase of wind with altitude, while for much higher 

 altitudes the one-fourth or other power might be preferable. The constant factor, 

 6 per cent, that enters this formula will of course not be understood as applicable to 

 other gauges or velocities or qualities of precipitation than those included in the above 

 table, but the close agreement of the computed percentages of deficiency shows that 

 we appear to be on the right track, and that some method must be devised by which 

 to free rainfall measures from the influence of the wind at the mouth of the gauge. 

 We see, in fact, that the simple wind-gauge which we have trusted so long is liable 

 to systematic error, whose magnitude is really enormous as compared with the small 

 errors that we ordinarily investigate in connection with thermometers, barometers, 

 and anemometers. 



ELIMINATION OF ERRORS OF THE RAIN GAUGE. 



Two methods are open to us by which to eliminate this error of the rain gauge. 

 One is instrumental, the other observational. 



Instrumental methods.— As before said, Profs. Bache and Henry seem, from their own 

 observations, to have clearly apprehended the nature of the error with which the gauge 

 is affected, and the latter was quick to suggest the remedy, namely, to so construct a 

 gauge that it shall closely imitate the conditions of the normal exposure, or that of a 

 gauge whose mouth is on a level with the ground, and which is, therefore, not cov- 

 ered over by the disturbing swift currents and eddies. The records of the Smith- 

 sonian show that Henry caused numerous experiments on this subject to be conducted 

 after he and Espy, in 18*48, inaugurated the Smithsonian system of meteorological 

 observers. In the second volume of Henry's collected writings, the reader can 

 easily consult his discussion of the erroneous explanations and his own correct ex- 



