322 EEPORT— 1893. 



observations, it was concluded by a careful investigation that at Wilhelms- 

 haven a rise of temperature of only 1° C. produces exactly the double 

 effect which is caused by an increase of barometric pressure of 1 mm., 

 viz., a motion of 0"-58 towards the east. 



Thus it is seen that, when these two meteorological agents happen by 

 chance to act in the same sense, very considerable deflections of the 

 plunib-line are produced at "Wilhelmshaven, which, in case they should not 

 be limited to the upper layers of the ground, would not escape detec- 

 tion at an astronomical observatory where regular observations are 

 taken. 



(8) A similar investigation led to the following results for Potsdam. 

 Barometric pressure appears to have no effect whatever on the position 

 of the pendulum at this place. But a rise of temperature of 1° C. pro- 

 duces a deflection of 0"-16 towards the east. This latter constant is 

 again determined witli considerable accuracy, the result of thirty-eight 

 days in April and May being 0"-18, whilst fifty-seven days of the later 

 period give the value 0"'15. 



(4) At Puerto Orotava the meteorological effects are small, but they 

 pi-esent a particular interest. They are not so easily noticed as in the two 

 former cases, where to see them 'it is sufficient to look at the diagrams. 

 Nevertheless they exist, and the following values could be deduced by 

 properly grouping the observations. A change of -f-1 mm. in barometric 

 pressure causes a deflection of 0"-0309 of the plumb-line to the west, and 

 a change of -|-1° C. in temperature a deflection of 0"-0362 to the east. 



The effect of barometric pressure is especially interesting, because, 

 from the description of the locality given above, it may be gathered that 

 it could be accounted for by the counteraction of volcanic forces, which 

 undoubtedly are still active inside the Pico de Teyde. If it is allowable 

 to draw any conclusions from the intervals of time in which during the 

 latest centuries volcanic action has shown itself on the outskirts of this 

 famous mountain, the time would now have arrived when the inhabitants 

 might be prepared to see another of those outbursts. It is not improbable 

 that, before an eruption takes place, the whole mass might show an 

 inclination to yield to variations of external pressure. And this is really 

 the case, for Puerto Orotava is situated on theKKB. flank of the cone, at 

 a distance of about 18 kilometres; thus, when the external pressure 

 diminishes, the slope of the mountain is increased, and a horizontal 

 pendulum placed as it was at Orotava ought to move towards the east. 



(5) It is interesting to compare the effects of temperature, which we 

 have now found to exist, with each other as well as with the daily oscil- 

 lation. If we divide the mean values of the latter by the mean corre- 

 sponding values of the maximum oscillation of temperature, we obtain the 

 following table, which corresponds throughout with a change of 1° C. : — 



Thus it is evident that, if it is the same cause which produces the 

 rnotion of the zero-point and the daily oscillation, this cause acts in a 

 difi'erent way and not in the same intensity everywhere. It is necessary 



