176 FOREST INFLUENCES. 



draw couclnsions fiaer than the data will warrant. To this end the law of errors 

 or the calculus of jjrobabilities must be appealed to. My present study relates prin- 

 cipally to subjects of fundamental importance, namely, the accuracy of the indi- 

 vidual statistics and the variability of distribution of rainfall chronologically and 

 geographically. 



RELATION Oi" P.VTTERN AXD ALTITUDE OF GAUGES TO ACCURACY OF RAINFALL 



MEASUREMENTS. 



With regard to the accuracy of rainfall measurements viewed simply as com- 

 parable data, two matters have been studied experimentally, namely, the size and 

 style of the gauge and the altitude above ground. With regard to size it is satis- 

 factorily shown that no error of more than 1 per cent systematically attaches to 

 gauges of the ordinary forms and of diameters ang^here between 4 and 44 inches. 

 With regard to the altitude it must be conceded that for a hundred yeai's it has been 

 known in a general way that observations by gauges nt various heights above the 

 ground are not comparable with each other. The remarkable influence of altitude 

 was first brought to the attention of the learned world by Heberden, who, in a me- 

 moir in the transactions of the Royal Society of Loudon, in 1769, stated that a gauge 

 on Westminster Abbey over 1.50 feet above the ground caught less than half as much 

 as a gauge at the ground. Since his day numerous others have instituted similar 

 observations in their respective localities. Usually they have been satisfied with 

 observing only one or two elevated gauges, but t>f late years, in order to fully eluci- 

 date the subject, more elaborate measurements have l>eeu made; thus Phillips and 

 Gray, at York, England, have observed at eight difi'ereut altitudes including the 

 gauge on the tower of York Minster. 



Rache, at Philadelphia, observed four gauges on top of a s(i[uare tower, and four 

 others on poles above them; Col. Ward, at Calne House, Wiltshire, observed ten 

 pairs of gauges at elevations of 20 feet or le.ss, each jtair consisting of an S-iuch 

 3,Dd ^ 5-inch gauge; Bates, at Castleton Moor, similarly observed ten pairs of 

 gauges ; Chi'imes, at Rotherham Reservoir, six gauges, at elevations of 25 feet or less ; 



( ?) at Hawsker, four 3-inch gauges, at altitudes of 10 feet or less; Wild, at St. 



Petersburg, six 10-inch gauges, at altitudes of 5 meters or less, and one at an altitude 

 pf 25 meters. A very laborious series of six or eight gauges at altitudes of 40 feet 

 pr less has, to ii^y knowledge, been carried on for some years by Fitzgerald, at Chest- 

 nut Hip, near Boston, but the results are not yet published. 



J^ will be seen, therefore, that abundant observational data are ;^t haml for the 

 elucidation of the peculiarities of the rair^ gauge, and the results that can be deduced 

 from sucl^ data command our immediate attention. Whatever uiystery has hitherto 

 attached to the undoubted fact that elevated gauges catch less rain is now fully 

 explained away. This phenomenon is of the nature of an error in the rain gauge 

 dependiiig upoi^ the force of the wind that strikes it, and as will be seen, now that 

 ^he knowledge of the source of error has Ijoen established, the njethod of correcting 

 Pf preventing it becomes simple. 



Jt will be reuiembered that Benjau\iu Franklirt, upon reading Heberden's memoir, 

 at once, in 1771, in ^ letter to Percival explained his results by the hypothesis that 

 falling cold rain drops condense the moisture they meet with in the warmer lower 

 strata, and that Phillips, ii; 1831, independently revived this hypothesis as explaining 

 the increase of rainfall. A mnch truer explanation had been suggested by Meikle, 

 in the Annals of Philosophy for 1819, and by Boace (Annals of Philosophy, 1822), 

 to the effect that the deficiency is due to the velocity of the wind and to the fact 

 that the gauge stands as an obstacle to the wind; also Howard showed that the 

 strength of the wind affected the higher gauge. But these minor notices seem to 

 have produced but little effect among meteorologists, and it remains for W. B. .Tevons, 

 Phil. Mag., 1861, vol. xxii, to demonstrate that the Franklin-Phillips hypothesis 



