154 METEOROLOGICAL OBSERVATORIES. 



below the freezing point, the wet-bulb record will show a fall of a 

 degree or two below the freezing point, and come back to it with a sud- 

 den leap when the wetted bulb has acquired a coating of ice, after 

 which the record again begins to show the true variations due to 

 evaporation from the frozen surface. 



Here there are the solar and grass radiation thermometers for meas- 

 uring the solar and the night radiation to the sky, and an ozone box 

 for registering the amount of that form of oxygen in the air, notably 

 very little in amount in the neighborhood of a great city. Observa- 

 tions of the water of the river Thames were made for a great many 

 years, the results being included in the Greenwich annual volumes. 

 In late years these were made at Deptford, but the series came to an 

 end in 1891. It seems a pity that some other points in the neighbor- 

 hood can not be found for continuing such observations. 



There are various rain gauges placed on the ground near the ther- 

 mometer screens, one on the top of the photographic thermometer 

 shed, one on the top of the magnetic house, one on the roof of the old 

 building fronting the park, and two at the Osier anemometer, one of 

 these being self-recording. These two latter are 50 feet above the 

 ground, and record only about three-fifths of the amount registered on 

 the surface of the ground below. The self-recording rain gauge acts as 

 follows: The rain is received in a vessel suspended by spiral springs. 

 As the vessel becomes heavier it sinks with greater or less rapidity, 

 according to the rate of the fall of rain. By means of a cord passing 

 therefrom, over a pulley to a pencil, this rate of fall is recorded on a 

 sheet of paper driven by clockwork. When 0.25 inch has been col- 

 lected, the vessel automatically empties itself, the pencil returns to 

 zero and begins again to record. The amount of rain falling during 

 any given interval of time is readily ascertained. 



With regard to the barometer, eye readings of the standard barome- 

 ter are taken, and the variations of pressure are registered by 

 photography. 



The standard barometer is a plain-looking instrument, of Fortin pat- 

 tern, made by Newman, and tne tube has a bore of 0.565 inch. It is 

 fitted with lamps and ground-glass screens to assist the readings. In 

 the year 1877 a very elaborate comparison was made by the late Mr. 

 Whipple between this barometer and the Kew standard barometer, 

 with the result that the difference between the two was found not to 

 exceed .001 inch, corresponding to a difference of only about 1 foot of 

 level, or to that caused by a few grains of impurity in the mercury — 

 one much less than the usual difference of reading as made by any 

 two observers. For photographic registration a siphon barometer by 

 a simple and effective plan, acting from a float in the shorter leg, is 

 made to raise and depress a slotted screen, so that by a clockwork 

 and photographic arrangement, similar in principle to that already 

 described for the thermometers, a permanent pressure curve is con- 



