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WRITINGS OF JOSEPH HENRY. [1855- 



of rain falls annually than at a less height on an extended 

 plain. The effect however, to which we now refer, is just 

 the reverse, since it is found that less rain falls on the top 

 of a tower, and even of an ordinary building, than at the 

 bottom. This phenomenon is due in part at least to the 

 fact that a drop in its descent through a foggy atmosphere, 

 in which the rain is falling, catches in its path all the min- 

 ute particles of water between the upper and lower stations. 

 It cannot be due, except in a slight degree, to the condensa- 

 tion of the transparent vapor in the atmosphere which oc- 

 cupies the line of its descent, since the condensation of this 

 would rapidly heat the drop of water, although its tempera- 

 ture were considerably lower than that of the air, on account of 

 falling from a colder region. The principal cause of the dif- 

 ference is to be found in the effect of the wind in passing over 

 and around the edifice on which the gauge is placed. The 

 effect of this cause was first investigated by Professor Bache, of 

 the United States Coast Survey, who made a series of obser- 

 vations with a number of gauges placed on different sides of 

 the roof of a shot tower in Philadelphia. He found that dif- 

 ferent quantities of rain were collected by gauges thus placed. 

 To explain the effect of the wind, we may refer to what 

 takes place when an obstacle like that of a large stone is found 

 with its upper end just below the surface of a running stream. 

 The water of the current will pass over and around the stone, 

 iind will rise above the general surface; there will exist a ten- 

 dency to a partial vacuum on the sheltered side; the liquid 

 in passing over and around the stone will be accelerated ; 

 the particles of water which pass around the stone, sup- 

 posing it to be a cylinder, will traverse a space equal to the 

 semi-circumference of the circle, while those moving along the 

 general current, and not deflected, will pass through a space 

 equal to the diameter of the same circle. A similar effect 

 would be produced by the wind striking against a tower. 

 The portion which passes around the top will be accele- 

 rated ; that which strikes against the top will be deflected 

 upward, and in both cases a diminution in the quantity of 

 jain which falls on the top of the tower will be the result. 



