COMPARATIVE METEOROLOGICAL OBSERVATIONS. 



233 



to the air, but sheltered from the sun and from rains, and is adjusted to a level. The 

 evaporating bowl has an area of one Paris square foot, and is about three inches deep. 

 Observations cannot be made when the temperature is below the freezing point. Full 

 details of the arrangement and use of this instrument were first given in the weekly 

 report of the Munich Observatory, No. 158, 1868. 



[Before this instrument was introduced, a simpler form of apparatus was used for 

 determining the amount of evaporation. It consisted of a broad shallow zinc vessel, of 

 known area, deepest in the middle, where it was connected with a discharge-pipe, 

 closed by a cock. Being filled by a measured quantity of water, it was exposed to the 

 open air, and the water drawn off and measured, from time to time, gave the amount 

 of evaporation. A still simpler form was a shallow pan, of known area, tilled with 

 water and weighed from time to time, or supported by a spiral spring, which gradually 

 arose, carrying an index, as the weight became less.] 



These instruments, in whatever form, give very dififerent results, according to the 

 material of which the pan is made. If of metal, it may become warm in greater or 

 less degree, according to the conducting power of the metal used. This result, how- 

 ever, becomes less important, where all the instruments used in the comparative 

 observations are alike in area, material, and exposure. 



Evaporation from soils. — The instrument for showing the amount of evaporation 

 from the surface of soils entirely bare, or covered with wood-litter, moss, or leaves, &c., 

 was constructed as follows : 



A box, A, is made of zinc, one Paris foot square, and 8 inches deep. At 2 inches 

 above the real bottom there is placed a perforated plate. An open cylindrical zinc 



vessel, B C, is connected with the 

 box by a tube. Into this there is slid 

 another vessel, C, a little smaller, 

 which is closed at the top, and is 

 furnished with a valve at the bot- 

 tom, opening upward and attached 

 to a spindle, as shown by the en- 

 larged figure. When in position, 

 this valve should stand level with 

 the perforated bottom of the box. 

 A thin mat of straw is first laid over 

 the perforated bottom, 

 and the box is then filled 

 with the same kind of 

 soil as that which is 

 found at tlie station, 

 either left with the sur- 

 Instrument for measnring the evaporation from soils. face bare or covered with 

 moss, leaves, or other substances, according as it is desired to measure their 

 effect upon evaporation. When the instrument is used the closed vessel, C, 

 is withdrawn, turned bottom upward, and filled through the opening at 

 the valve ; then, drawing the valve up with the fingers to close it, the ves- 

 sel is turned into proper position and placed in the open cylinder. The 

 valve rises as soon as the vessel full of water touches the bottom of the outer 

 shell, and the water flows in and fills the vacant space below and up to the 

 perforated plate. At this point the valve closes by the pressure of the water, 

 and the further access of air is stopped. The water will rise through the 

 soil from the perforated plate by capillary attraction and evaporate from 

 the surface, more being admitted by the automatic action of the valve 

 as the water below the soil gets below its level, a little air at the same 

 time getting into the closed reservoir; and it will thus continue to operate, 

 the soil being always uniformly damp as long as any water is left in the 

 reservoir. 



5. The earth thermometer. — For purposes of forestry, it is suflScient 

 to determine the temperatures at the depths to which roots penetrate and 

 spread their roo?s, and for this purpose observations are made at depths of, 

 i, 1, 2, 3, and 4 feet by thermometers snuk to those depths,^ and arranged 

 so that they may be easily withdrawn for observation. 



/:r\ 



Earth Ther- 

 mometer. 



1 In some cases thermometers, with stems several feet long, have been placed in the 

 earth, which allowed of readings without withdrawing them from the soil. The instru- 

 ments used at the Bavarian stations are the same as described by Dr. V. Lament, in 

 No. 19 of the Weekly Report of the Royal Observatory (Munich), and in the fourth 

 supplementary volume of the Annals of the Munich Observatory, p. 91, plate iv. They 

 differ from common thermometers by having the m.ercury in the bulb surrounded by 

 thick glass, which, being a slow conductor of heat, will not allow sensible changes to 

 occur when the thermometers are drawn up for observation. 



