526 SCIENTIFIC RECORD FOR 1683. 



by Dalton's, and quite as well as could be expected from the methods 

 of observation. The formula requires the accurate determination of the 

 temperature of the evaporating water and the velocity of the wind Jit 

 its surface, and applies, therefore, strictly to a freely exposed surface. 

 Evaporimeters established in protected places show great departures 

 from the computed values. In a second memoir Stelling shows that this 

 formula represents with sufficient accuracy the observations made upon 

 a free large surface of water, as in the case of his floating evapori meter, 

 elsewhere described. (See chapter on apparatus.) In this evapoi imeter 

 the temperature of the water in the surrounding river often differs by 

 one degree from that in the apparatus, and the latter is that required in 

 the formula, which reads as follows : 



»=A2(S-8) + B2(S-*)w. 

 Where w is the velocity of wind, S is the tension of vapor corresponding 

 to the temperature of water, s is the tension of vapor in the atmosphere, 

 ii is the amount of evaporation, A and B are constants, 2 is the sign of 

 summation. (Z. 0. G. M., xvn, p. 372.) 



Chistoni discusses the relative merits of two views as to the origin 

 and cause of dew that have sometimes been held to be antagonistic to 

 each other, viz, whether it is the moisture condensed out of the air that 

 comes in contact with surfaces cooled by radiation, as maintained by 

 Wells, 1819, or the moisture freshly evaporated from the earth and 

 plants during the night-time, and which, being too much for the air to 

 absorb, is carried in minute particles to neighboring surfaces that are 

 cooled by radiation and evaporation; a view apparently maintained by 

 Fusinieri, Zantedeschi, and Cantoni. 



By an extensive experimental investigation Chistoni demonstrates the 

 following points : 



(1.) Bodies that stand free in the air and have unrestricted radiation 

 into celestial space cool [as to their surfaces 1 ?] decidedly below the sur- 

 rounding air. [Chistoni's experiments seem to have been conducted as 

 a wholly independent repetition and continuation by newer instruments 

 and methods of those of Wells, Glaisher, Melloni, &c] 



(2.) Thermometers under, on, and above the surface of the ground 

 show that when dew is being deposited, the lowest stratum of air is 

 always colder than the ground below and colder than the air above. The 

 temperature of the air increases upwards [a confirmation of Fnsiuieri's 

 results]. 



(3.) The quantity of moisture evaporated from the earth and the lower 

 plants, and slowly ascending to be condensed as dew on the leaves of 

 higher plants, is very abundant. 



(4.) The influence of electricity claimed by Zantedeschi is not per- 

 ceptible. 



(5.) If we define dew as the aqueous precipitation that takes place 

 during the interval from sunset to sunrise, and is not rain or fog, then 

 its two chief causes are: (1) The cooling of bodies by radiation to tem- 

 peratures below that of the surrounding air. (2) The larger evapora- 



