106 



REPORT — 1895. 



(/) An Experiment on Evaporation. 



Because it was found that a load of about 1,000 lb., made up of men and 

 boys standing outside my observatory wall at a distance of 15 feet from 

 pendulum A, would deflect it 2 mm., the following experiment was made. 



In my garden a strong beam was rested on knife edges on the top of 

 a stake driven into the ground. On one end of this a box 1 foot 6 inches 

 square, and 6 inches deep, was hung, so that it could swing freely in a hole 

 cut in the ground. The box was tilled with earth which came from the 

 hole, and was covered with turf like the surrounding lawn. This load was 

 balanced by weights suspended at the other end of the beam attached to 

 which there was a pointer moving over a scale. During three fine days it 

 was found that the box lost weight at the rate of about ^ lb. per day per 

 square foot of surface, and as the surface of the material in the box was 

 similar to that of the surrounding ground with which it was level, it was 

 concluded that similar ground in the neighbourhood lost weight at about 

 the same rate. 



During a night the gain by precipitation of dew was sometimes as 

 much as 1'2 oz. per square foot. No doubt many accurate observations 

 have been made on the variation in the rate of evaporation and condensa- 



