234 



METEOROLOGICAL OBSERVATIONS. 



water by capillary attraction, thus keeping the bulb moist, which 

 causes the thermometer to indicate a temperature lower than the 

 dry bulb in proportion to the rate of evaporation ; whilst the dry 

 bulb shows the temperature of the air. The more rapid the 

 evaporation the greater is the difference of temperature shown 

 by the two thermometers. From this diii'erence can be calculated 

 by means of tables the amount of vapour in a certain volume of 

 air. 



Sunshine (Diagi-am 3). For many years the amount of 

 sunshine was only roughly estimated at this observatory, but 

 latterly it has been accurately determined by a Jordan Photo- 

 graphic Recorder, presented by Mr. John Davies Enys, F.G.8. 

 This instrument, which is placed on the roof of the Institution 

 where it has the full range of the sun at all seasons of 



JORDAN PHOTOGRAPHIC SUNSHINE RECORDER. 



the year, consists of two semi-cylindrical dark chambers, in 

 which are placed sheets of Cyanotype paper, one for the 

 morning and the other for the afternoon, ruled with vertical lines 

 indicating the hours and minutes of the day. The rays of 

 sunshine, which are admitted by a small aperture placed in the 

 centre of the rectangular side of each chamber, act chemically 

 on the paper, and travelling over it by reason of the earth's 

 rotation, cause a blue line to be left. The papers after being 

 fixed by immersing in water are dried and the trace of sunshine 



