428 Mr. W. S. Jevons on the Deficiency of Rain in an 



Prof. Bache's " Note on the Effect of Deflected Currents of Air 

 on the quantity of Rain collected by a Rain-gauge," communi- 

 cated to the British Association in 1838, is to a different effect. 

 It pi-oves experimentally the immense differences which may 

 occur between gauges placed at the different angles of a building, 

 but does not show why a gauge on the top of an obstacle must 

 on an average suffer a loss of rain. He found, however, that the 

 gauges to the leeward received in general more rain than those 

 to the windward, a fact fully in accordance with my theory. 



18. It is hardly necessary to add that my explanation has no 

 connexion with that of M. Flaugergues*, who in an unfortunate 

 moment mistook the sine for the radius of an angle, and argued 

 that " less rain will fall into the horizontal opening of the rain- 

 gauge when the rain is inclined than if it fell vertically, or in 

 a direction less inclined." As long as the drops fall in parallel 

 paths no such effect can be produced ; it is the divergence of the 

 rain-drops, owing to the varying velocity of the wind, which I 

 assert to be the cause of the deficiency. 



19. I will now approach the subject from an opposite point of 

 view, and show a priori that the real increase of rain between the 

 upper and lower gauges is not possible to any appreciable extent, 

 according to the only physical explanation of the phenomenon 

 which has ever been proposed. This theory was first suggested 

 by Benjamin Franklin f, who compared a drop of rain to a bottle 

 of cold water condensing dew upon itself when brought into a 

 warm room. That rain, even in our hottest days, he adds, comes 

 from a very cold region, is obvious from its falling sometimes in 

 the form of ice. 



This explanation has been repeated and adopted by almost all 

 who have expressed any belief in the phenomenon. But others 

 have shown its utter inadequacy ; and the single calculated ex- 

 ample given by Sir J. Herschel, in his recent excellent ' Essay 

 on Meteorology %,' may be adopted in our further discussions. 

 " Admitting," he says, "a given weight of rain to arrive at 213 

 feet from the ground, with the temperature of the region at 

 which it was formed unaltered, and supposing it to acquire in 

 the remaining 213 feet the full temperature of the air (both of 

 them extreme and, indeed, extravagant suppositions), admitting, 

 too (though hardly less extravagant), the mean height of forma- 

 tion of the rain to be 12,000 feet, it would bring down with it a 

 cold of 40° Eahr., which would condense (whether on the drops 

 or in saturated air if diffused through it) only 4 o ths, or Jj-th 



* Annals of Philosophy, vol. xiv. p. 114. 



t See his letter to Dr. Thomas Percivall, dated London 1771, in the 

 ' Memoirs of Thomas Percivall, M.D./ Appendix B. 

 t Page 104, as reprinted from the Encyclopedia Britannica, 8th ed. 



