180 



REl'ORT — 1897, 



Fig. 16.— Diurnal Waves at Shide, 

 1896. 



until the sun had passed the meridian, 

 or until 2 or 3 p.m. Then it returned, 

 following the sun until 7 or 9 p.m. 



The text accompanying these dia- 

 grams shows that the movements are 

 practically confined to fine days, from 

 which it may be concluded that the- 

 effect is connected with solar radiation. 



In previous reports I have suggested 

 that it might be produced by the differ- 

 ence in load removed by evaporation on 

 two sides of an installation, such loads 

 from a surface of grass being represented 

 by the removal of 4 or 5 lb. per square 

 yard per day. 



The experiment with the tarpaulin- 

 cover placed first on one side of the 

 tent and then on the other, which failed 

 to produce any marked effect on the 

 character of the diurnal motion, indi- 

 cates not only that this is practically 

 uninfluenced by differential evaporation 

 effects, but also by the heat received 

 by the ground on two sides of an in- 

 stallation, these effects being local. 

 We therefore have to look to the in- 

 strument, the pier on which it stands', 

 or external effects on a widespread 

 area. The fact that the diurnal wave is 

 marked on a brick pier rising from a 

 solid foundation in the middle of a 

 brick building shaded by trees,' and 

 also in cellars, in both of which places 

 the changes in temperature have been 

 small, indicates that the movements 

 are not to be accounted for by warp- 

 ings on the pier or portions of the in- 

 strument. 



The fact that strong and steady 

 westerly deflections corresponding to 

 an increase on the slope of the hill on 

 which T and V stood accompany wefc 

 weather, and that reverse movements 

 follow fine weather, indicates that a 

 load in the valley apparently causes 

 this to sink, whilst during the removal 

 of such a load it apparently rises. It 

 seems natural to conclude that the 

 diurnal waves are movements with a 

 similar origin. On hot days the valley 

 loses moisture, and therefore it rises. 



' Britixh Association BejJort, 1896, p. 213. 



