EVAPORATION AT PALERMO. 4'57 



OX THE EVAPORATION OBSERVED AT PALERMO Ix\ 1865 AXD 1860. 



By p. Tacchevi. 



\_ Translated for the Smitlisonian Institution from the Meteorological Bulletin of the Eoyal Ob- 



servatorij of Palermo.] 



Evaporation is one of the most important elements beaiing npon 

 climatological relations, but one which, until now, has been little studied. 

 In Italy there are some few stations in which such observations are con- 

 ducted, although by different methods. In the determination of this 

 element many difficulties and anomalies present themselves, insomuch 

 that it is regarded as the least certain of all the problems offered by 

 meteorology. Yet the instruments devised for the measurement of 

 evaporation have been of late greatly improved, and in our opinion that 

 of Vivenot is to be regarded as among the best. In our observatory the 

 evaporation is observed simultaneously with two atmometers, one of 

 which, being that of Gasparin, is entirely exposed to the direct action of 

 the air and the solar rays 5 the other, that of Vivenot, is stationed with 

 the other instruments of meteorology, designed for the temperature and 

 humidity, that is to say, is defended from the direct action of the wind 

 and sun. These two atmometers were described :n our Bulletins ]S"o8. 1 

 and 4 for 1865. The series of observations which we are about to pre- 

 sent extends through twenty months, from May, 1805, when a com- 

 mencement was made with the atmometer of Vivenot, to December, 

 18GG, inclusive. This interval is undoubtedly too short for arriving at 

 definitive conclusions; but on a comparative examination of the curves 

 of evaporation with those of the other meteorological elements, we have 

 always observed a constant relation, especially in regard to the tempera- 

 ture, force of the wind, and humidity, as may be verified by the monthly 

 reviews. In consequence of this, I propose to determine these relations 

 from the observations made, that is, to seek an expression for the eva- 

 poration in functions of those elements, whence a first approximation 

 may be obtained, serving as a contribution on our part to the researches 

 on the climate of Italy. 



[We are unacquainted with the atmometer of Vivenot j that of Gas- 

 parin consists of a shallow vessel of tinned copper, one meter square, 

 and fifty centimeters in depth. In one corner of this is a fine screw 

 placed perpendicularly, with a micrometer head at the top, and a fine 

 point at the bottom, by means of which a small fractional i^art of a milli- 

 meter of the depression of the surface of the water can be estimated. It 

 is also furnished with a maximum and minimum thermometer, the bulbs 

 of each being sheltered from the direct rays of the sun, while they par- 

 take of the temperature of the surface of the water. This apj^aratus is 

 freely exposed to the open air and sunshine. Care is required that no 

 water is allowed to fall into the vessel from rain, or if it does fall in, to 

 allow for the quantity. — J. H.] 



