138 REPORT — 1895. 



the continuity of the ground upon that side, which it is assumed in order 

 to produce the easterly deflection must rise. 



I put forward these conclusions simply as being, for the present at least, 

 the best that I am able to arrive at as explanatory of my own observa- 

 tions. The conclusions reached by Dr. E. von Rebeur-Paschwitz only 

 partly confirm my results. In the British Association Report for 189;j, 

 on p. 316, he says that ' the range of motion is on an average very nearly 

 p-oportional to either the quantity of sunshine or the maximum oscillation 

 of temperature during the day.' This and the fact that the movements 

 at TenerifFe, where the observatory appears to have been founded on and 

 surrounded by soil and rock, were very much more pronounced than they 

 were at Potsdam and Wilhelmshaven, where the soil was comparatively 

 soft, apparently support the view that the diurnal wave may be a distor- 

 tional efl'ect due to evaporation. On p. 320 of the same report, however, 

 he says that ' at Potsdam as well as Orotava the average range of daily 

 motion agrees most remarkably with those meteorological elements which 

 we may consider as a measure of the intensity of solar radiation. But I 

 must not omit to remark that the single days do not show this coincidence 

 equally well. For cloudy days occur with a large range of oscillation, and 

 clear days with a small range.' 



Although my observations in Japan have shown that when it was 

 cloudy and wet the diurnal wave has been absent, it is not impossible that 

 there may be cloudy days when, in consequence of wind, evaporation may 

 occur, and in consequence the daylight distortion may be marked. 



3. Effects due to Condensatiun{Xiy]it Effect). — It has been shown that 

 .at a favourably situated station the evaporation effect which has been 

 marked during the morning may late in the afternoon be the means of 

 starting a retrograde movement. It, however, remains to be explained 

 why a motion possibly commenced in this manner continues slowly during 

 the night until about 6 a.m. upon the following morning. Because this 

 movement is comparatively small it may be produced by the addition or 

 removal of a comparatively small load. 



The pi'ecipitation of dew, which on a uniform area like evaporation 

 follows in the wake of the sun, represents a feeble load, but the retrograde 

 motion continues when dew is not visible. But although dew may not be 

 visible, if we look beneatli a board which has been lying on the ground 

 .all night it is usually found to be very wet. This observation suggested 

 the idea that just as moisture is condensed beneath a board, so it may 

 be condensed in the ground within one or two inches of the actual 

 surface. 



During a hot day moisture is evaporated from the soil, which is per- 

 ceptibly heated to a depth of about one foot. Shortly after sunset the 

 surface to a depth of one or two inches is chilled or in winter it is frozen. 

 The result of this is, that moisture rising as vapour and by capillarity 

 from water-bearing strata is condensed on the underside of the chilled sur- 

 face. Like the dew we see on a uniformly covered surface the underground 

 dew should be first precipitated on the eastern side of a station and sub- 

 sequently ujDon the western side, and therefore during the night the surface 

 on the former side gains weight at an earlier hour than the latter. 



To determine how far superficial soils gain in weight by an action of 

 this description, independently of moisture precipitated from the atmo- 

 sphere or condensed as it rises out of the ground, the following experi- 

 ments were made. Two boxes each 1 ft. 6 in. square and 2 in. deeji were 



