312 EEPORT— 1893, 



moisture of the air in these cellars. The slightest difference of tempera- 

 ture was sufficient to produce dew on the glasses, and for a long time 

 during the months of May and June, when the temperature of the air 

 ■was rising quickly, observations became quite impossible. 



Observations at Puerto Orotava, Teneriffe. — The third station where I 

 was able to try the horizontal pendulum was at Puerto Orotava, on the 

 island of Teneriffe. The Spanish houses in this country do not have any 

 underground cellars, but through the kindness of an English lady, Mrs. 

 C. Smith, the widow of a well-known resident of the Canary Islands, 

 a small chemical laboratory on her grounds, which had been empty 

 for many years, was placed at my disposal. The estate bears the name 

 ' Sitio del Pardo,' and is well known to all English visitors of the place, 

 many of whom may have noticed the little laboratory, because it is 

 covei'ed with a most beautiful specimen of a white creeping rose. The 

 Sitio occupies the eastern flanks of an old lava stream, which runs from 

 the Montaueta de la Horca to the north towards the sea, and forms a 

 platform above the Puerto on which a large hotel is now built. 



The nature of the volcanic soil did not allow a similar foundation for 

 the instrument as was offered by the cellars at Potsdam and Wilhelms- 

 haven. Instead of making a hole into the ground I preferred to erect it 

 on the cemented floor of the little laboratory, which is on the same level 

 with the outer grounds, with the exception of the east side, where the 

 ground is higher. 



The little building, the direction of which very nearly agrees with the 

 magnetic meridian, is divided into two rooms, of which the one to the 

 south was used to place the hoi-izontal pendulum, whilst the one to the 

 north was reserved for the photographic apparatus. A door leading from 

 the former into the garden was closed by masonry work, and another 

 one connecting the two rooms received only the necessary openings for 

 the light to pass. Thus the pendulum was guarded as much as possible 

 against all disturbances. Observations were begun on December 26, 1890, 

 and continued without any interruption until April 27. Much trouble 

 arose from the bad quality of the petroleum. The lamps, which had 

 burned fourteen hours and more at the first two stations, had to be con- 

 stantly watched, and many hours of observation were lost. This, however, 

 was of no importance for the final reduction of the observations, because 

 the extreme regularity of the motions of the pendulum at this station 

 always permitted the gaps to be filled up by interpolation. 



The following paper, ' Das Horizontalpendel und seine Anwendung 

 zur Beobachtung der absoluten und relativen Richtungsanderungen der 

 Lothlinie,' • contains all the readings taken from the photographic curves 

 by the aid of a glass scale. 



Results. 



A preliminary study of the fir.st two sets of observations shows that 

 the investigation might be divided as follows: I. The influence of the 

 moon ; II. The daily oscillation and its changes ; III. The motion of the 

 zero-point ; IV. Seismological phenomena and others of a more incidental 

 character. 



I. The Influence of the Moon. — Since Professor G. H. Darwin and his 

 brother, Mr. Horace Darwin, made their well-known experiments at the 



' Nova Acta der Mis. Leap. Carol. Acad. Is. Nr. 1. 



