METEOROLOGICAL OBSERVATORIES. 155 



tiimously recorded. The slotted screen is carried by a lever of such 

 length that the scale on the paper becomes magnified to rather more 

 than four times the natural scale. This enables the finer oscillations, 

 as iu thunderstorms, to be better studied, and I may mention that this 

 barometer, in common with all other self-recording barometers through- 

 out the world, distinctly recorded in 18S3 the passage of the air wave 

 caused when Krakatoa, an island on the other side of the globe, was 

 rent in two by a volcanic eruption. 



On the top of the magnetic building we find the sunshine recorder, 

 of the Campbell-Stokes pattern, burning, by means of a lens, a trace 

 on a card in the usual way. Daily records of sunshine have been 

 maintained at Greenwich since the year 187G. Near the sunshine 

 instrument is a small, open, but well-protected thermometer screen car- 

 rying a dry bulb and maximum and minimum thermometers, for the 

 purpose of comparing the results at this altitude — 20 feet from the 

 ground — with the results of the standard thermometer at 4 feet above 

 the soil. 



On the top of the observatory tower, which has formed a landmark 

 since the time of Charles II, is found the Osier anemometer, for record- 

 ing the direction and pressure of the wind and the amount of the rain- 

 fall. The latter we have described in speaking of the rain gauges 

 generally. It is interesting to enter the little turret in which is the 

 recording table carrying forward a sheet of paper moved by clock- 

 work, and to watch the ever-moving pencils writing down at each 

 moment the direction and force of the wind, whether it is a mere breeze 

 or a fierce gale; a zephyr or a hurricane. The time scale usually 

 employed is about half an inch to an hour, but an arrangement now 

 exists by which, on a gale of wind springing up, the scale can be at once 

 increased to twenty-four times this amount, thus giving much more 

 minute information in regard to the variations of direction and pressure 

 at such times. This anemometer has been at work since the year 1811. 

 At a later date a Eobinson anemometer, for registration of wind velocity, 

 was added. It sometimes happens that the wind force is registered in 

 an unpleasant way at Greenwich, as elsewhere, by accident, and I show 

 you a picture of the shutter of the observatory dome which was brought 

 down by a gale on December 22, 1894. The pressure of the wind 

 measured at the time was 27| pounds per square foot, and the observer 

 inside the building narrowly escaped being struck by the suddenly 

 released counterweight used to balance the shutter. Mr. Nash has 

 favored me with these particulars. 



We should refer to the matter of the registration of atmospheric 

 electricity. For this the electrometer is employed, as designed by 

 Lord Kelvin (perhaps better known as Sir William Thompson). It is 

 placed in the principal room of the magnetic building, and consists of 

 a carefully insulated cistern, which is in communication with the elec- 

 trometer, and from which, by means of a pipe passing out into the open 



