534 SCIENTIFIC RECORD FOR 1883. 



rainfall with altitude: [The physical question is indeed thus far almost 

 entirely resolved into an instrumental one, i. e., what are the sources 

 of error, or how much are the rain-gauge records affected by local pe- 

 culiarities.] 



Jevons and Dines have shown that the wind eddies, due to the very 

 presence of the raiu-gauge and its support, largely affect the result. 

 The following points, however, may now be considered demonstrated : 



(1.) The ratio of the rainfall on a tower and on the earth depends 

 upon the direction of the wind. 



(2.) In a calm the rainfalls on a tower and on the earth are equal. 



(3.) For a given prevailing wind the rainfaH on the tower on the 

 windward side is smaller than on the earth, whilst on the leeward side 

 the rainfall is equal to or greater than that on the ground. 



(4.) The excess on the leeward side compensates the deficiency on the 

 windward side. 



(5.) On a very large roof the rainfall at the center is the same as on 

 the ground. (Z. 0. G. 31., xvn, p. 114.) 



Whipple lias published a discussion according to a simple method of 

 the question whether five or thirteen year periods are any ways ap- 

 parent in the long series of rainfall records at Paris, London, Milan, 

 &c. He concludes that no periods, and especially none so short as these, 

 are deducible. (Z. 0. G. 31., xviii, p. 47.) 



VII. — WINDS AND CURRENTS. 



Prof. A. Overbeck, of Halle, has published a highly important memoir 

 on the movement of the atmosphere on the surface of the earth. As- 

 suming the surface to be level and smooth and the frictional resistance 

 proportional to the velocity of the wind, he finds from the differential 

 equations of motion of an incompressible fluid conclusions relative to 

 the inclination of wind to the gradients, some of which had already been 

 given by Guldberg and Sprung. Overbeck gives detailed formulae and 

 computations for the inner and outer portions of a cyclone and anti- 

 cyclone, presenting conclusions not veiy different from those of Ferrel 

 (Met. Res., Part 2), but of course strictly applicable only to the ideal 

 earth and atmosphere. (Z. 0. G. 31., xviii, p. 106.) 



Helmholtz' scientific memoirs having been reprinted, Hann calls re- 

 newed attention to a memoir of 1873 " on a theorem relative to geomet- 

 rically similar motions of fluid bodies, &c." Starting with the statement 

 that we have long known the correct differential equations of fluid mo- 

 tion, but cannot generally obtain the integrals therefrom, Helmholtz 

 shows that for a large class of motions, where the compressibility of gas 

 or liquid underpressure does not affect the phenomena, the laws of mo- 

 tion in gases are similar to those in far more incompressible fluids, and 

 the motions on a large scale of a very compressible gas or liquid are 

 similar to those on a small scale and with small velocities of a correspond 

 ingly less compressible fluid. Similarly with the friction, its effect is also 

 less important in motion on a large scale; in fact, in experiments on large 



