62 REPORT 1861. 



observed to follow ; and also that the internal pressiu-es were exactly such as were 

 consistent with Dr. TjTidaU's views of the cause of the laminar structure of glacial 

 ice, so far as it was a necessary condition, according to those views, that the stnic- 

 tm-al lamina shoidd be perpendicular to the dii-ection of maximum pressiu-e. 

 Moreover he showed that the internal action was inconsistent with Principal 

 Forbes's theory, which attributed the laminar structure to a diiFerential motion of 

 the contiguous lamina. He also explained the importance of the slidinff of glaciers 

 over the beds of their containing valleys, not only as the cause of a large portion of 

 the whole observed motion, but also as increasing in a large degi-ee the efficiency 

 of the internal tension and pressiu-e in producing the dislocation and crushing 

 which were necessaiy for the general motion of the glacier. 



On the Deficiency of Bain in an Elevated Rain-gauge, as caused by Wind. 



By W. S. Jevons, B.A. 



When wind meets any obstacle, those strata of air which are near to the obstacle 

 must be compressed, and must move with gi'eater rapidity, just as a river moves 

 most quickly in the narrowest parts of its channel. Thu8,"wind blowing against a 

 house or tower has a greater velocity just above the summit of the building, than 

 where there is no disturbance. Now a rain-drop, when falling through the vdnd, 

 describes the diagonal of the rectangle of which the sides represent the velocities 

 due to gravity and the impidsion of the wind. The path of a rain-drop then is in- 

 clined at an angle from the vertical direction of which the tangent vanes nearly as 

 the velocity of the wind. Two equal rain-di-ops, therefore, falling into a cun-ent of 

 air at points where the velocity is not the same, wiU not pmsue parallel paths. The 

 one drop will either approach to or recede from the .other, and the eflect will be to 

 increase or diminish the quantity of rain falling in the intemiediate space. A dia- 

 gram or a slight calculation will show this eflect to be considerable, so that when 

 a shower of rain falls through wind upon any obstacle, such as a house, a large part 

 of the rain-drops wUl be blown beyond the obstacle by the increased velocity ofthe 

 ■wind, and less rain wiU fall on the windward part of the top of the obstacle than 

 elsewhere. 



An ordinaiy rain-gauge, even when suspended in mid-air, will likewise act as an 

 obstacle in the same manner, but in a less degree. Until, then, this effect of the 

 wind upon the amoimt of rain collected in a gauge, either suspended in the air or 

 placed upon a building, be properly allowed for, no conclusion can be drawn from 

 any rain-gauge obser\-ations as to a real vai-;ation ofthe rauifall according to elevation. 



Observations by Luke Howard, by Boase of Penzance, and by others clearly exhibit 

 this influence of the wind. Other published observations, even those at the Green- 

 wich Obsei-vatory, exhibit such gi'eat and uregidar individual discrepancies, that no 

 valid conclusion can be drawn from them. To take an average under such circimi- 

 stances, it is argued, gives a purely fallacious appearance of unifomiity and law. 



The possibility of a real variation of rain with elevation is then treated on a priori 

 gi'pimds, and it is concluded that the condensation theory, fii-st proposed by Benja- 

 min Frankhn, and the only one of the least validity ever oifered to account for the 

 apparent variation of rain, will never, imder the real chcumstanc es of the atmo- 

 sphere, account for more than an almost infinitesimal increase of the raiu-di-ops in 

 the last few hundred feet of descent. 



Observations on the coldness of rain when it reaches the surface oppose, instead 

 of supporting, the theory of condensation, since the colduess of the rain-drop proves 

 that it has condensed little or no vapour throughout its descent. 



Arago's argument in favoiu- of the increase of rain-di-ops near the surface, founded 

 on the disappearance of the supernumerary rainbows near the ground, is also quite 

 inconclusive, since the rain-drops probably become irregidar in size by coalition, and 

 would not become iiregular by condensation of vapour. 



All observations by rain-gauges much elevated or exposed to wind ai-e to be 

 rejected as fallacious, and in accm-ate rain-observations it is recommended to place 

 a verj' flat collecting vessel of considerable ai-ea in the centi-e of a flat surface upon 

 the gi-oimd, in an open place, so that no appreciable obstacle may be opposed to 

 the wdnd, and the splashing of the rain-drops within and without the collecting 

 vessel may compensate one another. (See Lond. & Edinb. Phil. Mag. Dec. 1861.;) 



