240 



MESSRS. R THRELFALL AND J. A. POLLOCK 

 NATURAL Minimum. Sydney, December 18th, 1898. 



By far the greater number of our observations have been made by artificially heating 

 the lower part of the case by placing a batswing gas burner, or a lamp, on the floor 

 directly under the centre of the Kew magnetometer tripod, with a view of getting 

 the temperature of the box between 2 and 3 above that of the air of the 

 room. The source of heat is taken away after about three-quarters of an hour, and a 

 maximum reading is obtained within the next hour. It is surprising that such 

 barbarous treatment as heating the instrument from beloiv only, with a naked flame, 

 should have given results worth recording, but from the plots of the observations 

 taken in this way it will be seen that they are most accordant. They will be 

 referred to as artificial maxima. The success of this method of observing depends 

 entirely on the perfect freedom of expansion of every part of the instrument. For 

 reasons, which we give afterwards, we now only observe either natural maxima or minima. 



ARTIFICIAL Maximum. Hornsby, December 15th, 1898. 



