156 METEOROLOGICAL OBSERVATORIES. 



air, a small jet of water is projected into the atmosphere. The electric 

 potential of this point is thus communicated to the electrometer and 

 recorded by a. photographic arrangement on a revolving cylinder. In 

 fine weather the electricity is usually positive in the air as compared 

 with the earth, but in rainy weather, or in thunderstorms, rapid 

 variations from positive to negative and back again are experienced. 



Going now to the basement of the building we find the apparatus 

 devoted to the registration of the delicate movements of magnets, 

 made of hardened steel and delicately suspended by long silk filaments 

 or moving on fine knife edges. Each magnet carries a small mirror so 

 arranged as to reflect a spot of light from a lamp on to a piece of 

 sensitive paper placed on a cylinder turned round by clockwork, so 

 that every variation or tremor of the magnets is recorded by a corre- 

 sponding varying line on the moving paper, making waves of more or 

 less steepness thereon, according to the amount of movement. 



When there is no movement or disturbance the line is straight, but 

 this is not the usual state of things. It is scarcely necessary to remind 

 you that any vibration, as from an earthquake shock, may also disturb 

 the magnets mechanically, but earthquakes are rare in this country 

 and it has not been thought necessary to set up special apparatus 

 more particularly designed for the registration of such phenomena. 

 On February 23, 1887, I find from the Astronomer-Royal's Report the 

 vibration caused by an earthquake as far distant as the south of 

 France caused a disturbance of the magnet corresponding to 20' of arc 

 in declination and 0.004 of horizontal force, being one two hundred and 

 fiftieth of the whole horizontal force. 



There is also the earth current apparatus for registration of the gal- 

 vanic currents, that to a lesser or greater extent are always j)resent 

 in the earth. Two wires, each several miles in length, and both having 

 earth plates at the two ends of the line, are placed in communication 

 with galvanometers, one to each circuit, each galvanometer carrying a 

 mirror for photographic registration of the variation of each current 

 force on one cylinder placed between the two galvanometers. Since the 

 end of the year 1890 these records have been greatly disturbed during 

 the day by the trains running on the City and South London Electric 

 Eailway, although the nearest earth plate of the system is distant some 

 2J miles from the railway. 



As a concluding remark we may mention that the time scales of all 

 the records throughout the observatory, both magnetical and meteoro- 

 logical, are, with one exception, identical in length, which much facili- 

 tates any collation of the various registers. The one unavoidable 

 exception is tbe sunshine record, which has a somewhat more extended 

 scale. Those similar in length number in all thirteen. 



I have given these somewhat minute particulars of the work done at 

 Greenwich to serve as an example of that which goes on at all observa- 

 tories of the first order throughout the world, varying a little under 



