320 



REPORT 1893. 



We see from these numbers that at Potsdam as well as Orotava the 

 average range of daily motion agrees most remarkably with those meteoro- 

 logical 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. 



At Wilhelmshaven, though we find that a similar relation is indicated, 

 it is not so well marked. If we neglect the first three and the last three 

 lines in the above table, which rest on a small number of days only, the 

 range of motion appears to be very nearly constant. It is this circumstance 

 which allowed the lunar wave to be determined with much more accu- 

 racy than in either of the two other cases. 



The above results make it evident that the daily oscillation is to a 

 great extent due to thermal efi'ects of the sun. Probably there are other 

 causes acting besides, but temperature and radiation are certainly the 

 principal agents. Owing to this fact it would be quite useless to try and 

 study the tidal effects of the sun. 



How this thermal effect takes place remains as yet an open ques- 

 tion. It seems quite impossible to connect the observed oscillation 

 with purely local influences, such as we constantly observe in our 

 astronomical observatories. It would certainly be a most extraordinary 

 chance if the radiation of the sun should produce such similar effects 

 on the foundations in three places, so entirely different from each 

 other respecting all conditions. I may mention besides that a few 

 observations made at Karlsruhe in 1887 and recent observations by 

 Professor Kortazai at Nicolaiew show the general character of the 

 curve to be the same in those two places as we have found it. Until 

 further researches have been made, we may therefore consider the 

 daily oscillation of the plumb-line as a general phenomenon all over 

 the surface of the earth, and consequently it would afi'ord great in- 

 terest to study it at a number of places equally distributed over it 

 and with as varied conditions as possible. 



The question naturally arises whether these motions which we must 

 consider as motions of the soil are still noticeable at a certain depth 

 below the surface. We ought not to expect this, according to what 

 has just been said ; but, on the other hand, the law of continuity makes 

 it difficult to understand how they should disappear quite near the sur- 

 face, at a depth down to which the daily change of temperature pene- 

 trates, when they are still so noticeable in such a place as the cellar of 

 the Potsdam Observatory. 



I have had no chance until now of trying this interesting experi- 



