l.'JS Mr. T. Tate on a new Evaporameter and Absorbometer. 



after the lapse of any proposed time, the evaporation from the 

 surface of the water in the glass bell will have caused the water 

 in the tube to have moved towards E ; and this distance con- 

 verted into cubic inches, by a relation of scale previously ascer- 

 tained, will for short intervals very nearly give the amount of 

 evaporation ; but this will in all cases be more accurately and 

 directly found by depressing the displacement-gauge until the 

 liquid in the tube is brought back to the mark a ; for the read- 

 ing of the mark on the gauge, coincident with the mark on the 

 glass bell, at once gives the number of cubic inches of water eva- 

 porated in the interval of time, care being taken to give a slight 

 oscillation to the w r ater in the tube, as already explained, before 

 the reading is taken. After a series of observations it will be 

 found that the displacement-gauge has nearly reached its lowest 

 point of depression ; in this case fresh water must be added, and 

 the adjustment made as already described. If rain has fallen 

 during the interval of observation, the amount of this rain, as 

 determined by an ordinary rain-gauge, reduced (if necessary) to 

 the surface of the water of the glass bell, must be added to the 

 amount of evaporation indicated by the evaporameter, and then 

 the excess of this quantity over that which has flowed into the 

 tube T will give the true amount of evaporation. Should the 

 water in the tube extend beyond the mark a, the gauge must 

 in this case be depressed so as to throw a portion of water into 

 the tube T, and then the process may be conducted as above 

 described. The amount of evaporation thus determined is that 

 which is due to the surface of the water in the glass bell ; but by 

 proportion the evaporation due to any other surface may be 

 readily found. The water-surface of the instrument which 1 have 

 constructed is equal to about 80 square inches, and the inclina- 

 tion of the tube is about 1 in 50 : when the inclination is much 

 less than this, the motion of the water in the tube becomes 

 somewhat fitful and irregular. 



With the view of testing the reliability of the instrument, I 

 have compared the results derived from it with those derived 

 from the balance, and have found that they nearly agree with 

 each other. Thus the evaporation during twenty-four hours was 

 found by the instrument to be 2*2 cubic inches, whilst the eva- 

 poration during the same time, as indicated by the balance, was 

 found to be 540 gr. or 2*14 cubic inches. 



No doubt the variation of the force of liquid cohesion due 

 to change of temperature will form a source of error in the read- 

 ings of this instrument ; but this variation, slight even at extreme 

 atmospheric temperatures, must be exceedingly small for any 

 change of temperature which can take place between two conse- 

 cutive observations. 



