100 Report of the Kew Observatory Committee, 



similar lines, and in particular comparisons have been made of the 

 potential at the spot where the jet of the water-dropper breaks up 

 with that at a station outside^the immediate influence of the Observa- 

 tory buildings. 



Aneroid Barometers. — A large number of experiments have been 

 made in continuation of those mentioned last year, and the readings 

 have been reduced. Further experiments are contemplated. In the 

 meantime the results are being worked up by the Superintendent. 



Nocturnal Radiation. — The observations with minimum thermo- 

 meters referred to last year have been continued. 



Platinum Thermometers. — Observations have been made from time 

 to time of the readings of six platinum thermometers in ice and 

 steam, and of certain of the number in sulphur vapour and in molten 

 silver, special attention being paid to changes of zero. 



A series of comparisons of mercury and platinum thermometers 

 at temperatures above 100° C. has been made, the thermometers 

 being immersed in a well-stirred fusible metal bath, kindly lent by 

 Mr. C. T. Heycock, F.H.S., and Mr. Neville. The comparisons were 

 initiated by these gentlemen, who visited the Observatory during 

 several days in January. 



Early in the year a platinum resistance thermometer was buried 

 underground along with two iron resistance thermometers of a type 

 invented by Professor H. Callendar, F.R.S., and Mr. E. H. Griffiths, 

 F.R.S. During the year readings of earth temperatures have been 

 taken usually twice a day, and a report on the behaviour of the 

 instruments has been submitted to the Meteorological Council, on 

 whose behalf the experiments were undertaken. 



Small inconsistencies in the behaviour of the resistance box used 

 with the platinum thermometers having been brought to light, the 

 Committee have arranged with Dr. Harker, late of Owens College, 

 Manchester, to investigate the matter, so that the defect may be 

 remedied. 



Photographic Test for Lenses. — Difficulties having been repeatedly 

 met with in applying the present tests for curvature, astigmatism, 

 and definition, experiments have been made which may lead to 

 replacing or supplementing these tests by direct observations and 

 measurements on a photographic plate. 



