ON SEISMOLOGICAL INVESTIGATIONS. 



103 



occurs two hours later than in the N.-S. recoixls, occurring in the latter 

 about two hours before the lamp is lighted. 



It appears from the above that with the seismograph exposed to the 

 ordinary diurnal range of temperatui-e the registers are affected either by 

 air tremors or by the lamp introduced to check them. For this reason, in 

 the month of July 190J: the instrument was removed to the magnetic 

 basement, in which the diurnal range of temperature is seldom greater 

 than 0°-2 (Fahrenheit), the chamber having double walls, with an eigh teen- 

 inch air-space between them, and its floor being 1*2 feet below the surface 

 of the ground. The chamber is ventilated by means of a 12-inch pipe laid 

 at a depth of 11 feet below the surface of the ground, and communicating 

 with the air at a point 35 yards distant. A hole was dug, 10 feet deep by 

 3 feet square, and hlled up with 7 feet of concrete, on which a tapering 

 column, also of concrete, was built up to a height of 3 feet above the floor 

 for the reception of the seismograph. 



The registers obtained with this installation show no air tremors 

 whatever. No measures have yet been made, but from an examination 

 of the photographs it would appear that the character of the diurnal 

 inequality of level has altered considerably. Its amplitude is very much less 

 than formerly, and the maximum tilt to east occurs at about 8h. in place 

 of at 17h., and the maximum northerly tilt at about 17h. in place of at 6h. 



The accompanying vector diagram (fig. 2) shows the mean diurnal tilt of 

 the boom for tlie year 1903, as determined from the hourly measures of 

 the north and east components. The effect of lighting the lamp (at about 

 5|h. P.M.) is to tilt the pillar towards the south, and thus reverse the 

 northerly tilt, which sets in after 14h., and to form a closed loop, but 

 there is no corresponding tilt to north when the lamp is extinguished at 

 6 A.M., the boom tilting steadily towards the south-east until 2 p.m. 



Fig. 2. — Mean Diurnal Inequality' of Level at the Royal Alfred Ohservatory, 

 Mauritius, in the year 1903. (^Mauritius Civil Tivie.) 



