TK.VNSACTIONS 01' THE SECTIONS. 35 



Heat. 



On the Absorption, Emission, and Rejlectlon of Heat. 

 By Professor Gvstav Magnus. 



[Printed in exiensQ fimong tUe Beports, see page 214.] 



Meteoeologt. 



On the Determination of tlie Beal Amount of Evaporation from the Surface of 

 Water. Btj Eogees Field, B.A., and G. J. Symons. 



The authors hegin by pointing out the extremely discordant results arrived at 

 hitherto by the highest authorities as to the amount of evaporation from a water- 

 surface ; one observer, for instance, giving tlie amount as 44 inches, and another 

 giving it as 11 inches for the same year. Some difference in the results might be 

 expected, in consequence of difference of locality, but such startling differences 

 can, it is believed, only be explained hy the very faulty nature of the evaporators 

 in common use. After giving a quotation from Professor Daniell's Meteorological 

 Essays, strongly condemning the ordinary evaporators, the authors proceed to cri- 

 ticize the mode of calculating the evaporation from hygrometric observations, pro- 

 posed by Professor Daniell as a substitute for quantitative measurements, and 

 come to the conclusion tliat this method is practicallj' useless. 



The great objection to ordinary evaporators is their diminutive size, in conse- 

 quence of which the water becomes unduly heated, and evaporation unduly 

 increased. The only published experiments on a large scale of which the authors 

 are aware are those made at Dijon, and other places on the Bm-gundy Canal, with 

 tanks eight feet square, and these gave an evaporation of only about half that 

 generally adopted for the district. Moreover, a small tank one foot square placed 

 by the side of one of the large tanks, gave an evaporation 50 per cent, greater than 

 that from the large tank. Some experiments on a smaller scale at St. Helena 

 also show the way in which rmdue heating increases the evaporation. In these a 

 small evaporator, fully exposed, gave 50 per cent, more evaporation than a similar 

 evaporator placed in a tub of water. 



Large tanks, like those at Dijon, can, however, only be used in exceptional cases, 

 and it therefore becomes important to devise some simple arrangements which 

 should give approximately correct results. The authors have recently commenced 

 some experiments on the subject, and desire to place upon record .some of the facts 

 arrived at. 



In these experiments the depth of water evaporated was ascertained by direct 

 measurement, without emptying the evaporators, as is generally done. This mea- 

 surement was efl'ected by means of a small instrument, called a " hook-gauge," 

 shown on the accojiipanying diagram. The point of the hook can be adjusted with 

 great precision to the exact level of the surface of the water, and the depth read 

 off on a vernier to the hundredth of an inch. By resting the clamped bar on the 

 top of the evaporator, the zero can be placed in any convenient position. 



The arrangements adopted by the authors are shown on the cliagram. Fig. 1 is 

 perhaps one of the best forms of ordinary evaporators, consisting of a copper 

 vessel fully exposed ; fig. 2 is an an-angement designed by Mr. Symons, wherein 

 the vessel, still of metal, is sunk almost wholly in the ground ; fig. 3 is a modifi- 

 cation of the St. Helena plan, consisting of a glass cylinder placed in the centre 

 of a much larger vessel of water. 



A detailed Table is given of the observations for three weeks, ending August 

 12, 1869. The chief residts are— 



(1) The total evaporation from fig. 1 was 4'37 inches, from fig. 2, 3'13 inches, 

 and from fig. 3, 2 '46 inches, numbers which are to each other in the 



