Meteorology. 



179 



differences of the decrements are nearly constant, beyond that temperature 

 the differences of the decrements also increase. 



4. Beyond the temperature of 100°, a portion of the power of the mag- 

 net is permanently destroyed. 



5. On a change of temperature, the greatest portion of the effect on the 

 intensity of the magnet is produced instantaneously, which proves that the 

 magnetic power resides on or very near the surface. 



6. The effects produced on unpolarised iron, by changes of tempera- 

 ture, are directly the reverse of those produced on a magnet, an increase 

 of temperature causing an increase in the magnetic power of the iron, 

 the limits between which Mr Christie observed, and they were 50° and 

 100°. 



12. Diurnal Variation of the Terrestrial Magnetic Intensity . — The fol- 

 lowing interesting table, given by Mr Christie in the paper above men- 

 tioned, shows the diurnal variation of the magnetic intensity in May and 

 June, according to his own observations, and those of Hansteen's : 



Intensity according to Hansteen's 

 Observations in 1820. 



Intensity according to Mr Christie's 

 Observations in 1823. 



13- Influence of Copper on the Oscillations of Magnetic Needles- — We 

 have already mentioned in our third number Mr Arago's discovery of the 

 influence of copper on magnetic needles which it enclosed. When a ho- 

 rizontal needle, suspended in a ring of wood by a thread or fibre, was 

 moved 45° from its natural position, it performed 145 oscillations before 

 the amplitude of the arc of oscillation was reduced to 10°- When the 

 needle was suspended in a ring of copper, and was moved 45° from its natu- 

 ral position, it only performed 33 oscillations before the amplitude of the arc 

 was reduced to 10°. In another ring of copper of less weight the needle 

 performed 66 oscillations before the amplitude was redueed to 10°« 



14. Effect of Copper in motion on a Magnetic Needle. — M. Arago has 

 more recently discovered, that if a plate of copper revolves under a mag- 

 netised needle contained in a closed vessel, the needle will deviate from 

 the magnetic meridian, the deviation increasing with the velocity of the 

 copper. If the velocity of the copper is sufficiently great, the needle will 

 turn continually round the wire on which it is suspended. — Ann. de Chim. 

 See this Number, p. 135- 



METEOROLOGY. 



15. Danield Improvement on the Barometer. — Mr Daniell has found 

 that air insinuates itself into the vacuum of the best made barometers, in 



