ELIMINATION OF THE EFFECT OF THE WIND, 181 



planation of the pheuomeuon. and at page 262 will be found Henry's suggestion of 

 " the shielded gauge." This shielded gauge was an ordinary small cylindrical 

 gauge; a few inches helow the mouth of this gauge a horizontal circular plate of tin 

 4 or 5 inches wide was soldered to it like the rim of an inverted hat; bj' this means 

 he hoped to ward off the disturbing eddies which would necessarily be formed almost 

 wholly beneath the flat rim and therefore harmless. 



Although Henry's shielded gauge was described at least as early as 1853, yet I 

 have not found as yet any records or observations made with it, though such 

 probably exist, as Henry's suggestion was widely distributed among the Smithso- 

 nian observers. 



In 1878 Prof. Nipher, of St. Louis, published the first results of his observations 

 with his own shielded. gauge, as independently invented by him. He surrounds the 

 upper portion of the gauge by an umbelliform screen made of wire gauze; the fall- 

 ingrain strikes on this and breaks up, and falls down to the ground without spatter- 

 ino- into the mouth of the gauge at the center, while the ganze sufficiently breaks 

 up the wind currents to maintain a normal condition of the air at the mouth of the 

 oauffe. Nipher's own experiments with this gauge showed that its catch at a height 

 of 118 feet above the ground was nearly the same as that of the ground gauge itself. 



The invention of the shielded gauge gives us the required instrumental solution 

 of our problem. Of late years Bcirnstein in Berlin and Wild in St. Petersburg have 

 experimented very largely with Nipher's shielded gauge and have reported in its 

 favor. Hellraann, during 1887, also observed with a Nipher gauge, and finds the 

 effect of the shielding to be very favorable, but not so much so as to make it quite 

 equal to the ground gauge. 



The good accomplished by the shields adopted by Henry, or by Niiiher, can also 

 be largely attained by a simple system of protection from wind. By " a protected 

 gauge" I mean an ordinary gauge whose mouth is a few feet above the ground, and 

 which is surrounded at a distance of a few feet by a fence or screen separate from 

 the gauge, and whose top is a little above the mouth of the gauge. The protecting 

 fence is therefore so arranged that it diminishes the wind at the mouth of the gauge 

 without itself introducing new and violent injurious eddies. Both Bcirnstein, Wild, 

 and Hellniann have experimented with such protected gauges, the protecting fence 

 being so constructed that the angular altitude of the top of the fence as seen from 

 the mouth of the gauge is between 20 and 30 degrees. The catch of the gauge thus 

 protected always exceeds that of the frfie gauge, so that the correction to reduce it 

 to the ground gauge is comparatively quite small, the deficit being reduced from 25 

 per cent down to 3 or 4, 



Hellmann has also made the following interesting experiment: The roof of the 

 Academy of Architecture in Berlin, where the Royal Prussian Meteorological Insti- 

 tute is temporarily domiciled, covers about 50 meters square, and is not merely flat, 

 but depressed considerably below the rampart walls of the building; it, therefore, 

 constitutes a grand protection to any gauge placed near tlie center of the roof, and 

 accordingly Hellmann finds that in this location gauges catch more than anywhere 

 else on the roof or the ramparts, and but little less than agaiige on the ground. His 

 conclusion is that the Nipher, or similar protection, can nearly, but still only partly, 

 annul the injurious influences of strong winds on the catch of the gauge. 



The reduction or correction of rainfall for altitude, as it has hitherto been called, 

 is therefore really a correction or reduction of the readings of the rain gauge for an 

 instrumental error due to the wind. 



Observational methods. — As an observational method of obtaining the true rainfall 

 from the gauge reading, aud if it is impracticable to establish a normal pit gauge in 

 a good location, or if it be desired to determine approximately the correction to be 

 api)lied to past records obtained from a gauge that still remains in the former place, 

 the following arrangement offers a fair api)rt)xiniation. 



Kthe present gauge has been standing in an open field at a few foet elevation, 



