ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 143 



crucible to about 100° C, and gently stirred until the zinc is completely 

 dissolved in the mercury. The amalgam is liquid while warm, and must 

 be poured into the cell before it becomes solid on cooling. 



The vessel containing the element consists of two vertical tubes. 

 These, as shown in the figure, are closed below and open above into a 

 common neck, which can be closed by a ground stopper of glass. The 

 two tubes should be 2 cm. in diameter and 3 cm. in length. The neck 

 should be at least 1-5 cm. in diameter and 2 cm. long. A short length 

 of platinum wire is sealed through the bottom of each tube. 



The end of the wire in one tube is covered by a small quantity of 

 pure mercury, that in the other tube by the zinc-mercury amalgam. 



Above the mercury a layer about 1 cm. thick of the mercurous 

 sulphate paste is placed ; above this, and also above the amalgam, a layer, 

 also about 1 cm. in thickness, of zinc-sulphate crystals, and the vessel is 

 filled up with the saturated zinc sulphate solution. 



The zinc-sulphate crystals are obtained by evaporating at a tem- 

 perature of less than 30° C. some of the zinc-sulphate solution prepared 

 as in 4 of the specification. 



The stopper is then inserted, leaving a small air bul^ble above the 

 liquid, and sealed on the outside with shellac dissolved in alcohol. 



The ends of the platinum wires outside the cell forua the two poles, 

 and should be connected to suitable terminals. 



The Application of Photography to the Elucidation of Meteoroloqienl 

 Phenomena. — Fourth Report of the Committee, consisting of' Mr. 

 G. J. Symons (Chairman), Professor R. Meldola, Mr. J. 

 HoPKiNSON, a7id Mr. A. W. Clayden (Secretary). (Braivn up hi/ 

 the Secretary.) 



In presenting their report on the work of the last year your Committee 

 have but little to say on the subject of the representation of clouds and 

 lightning by photography. They consider that their collection is nearly 

 complete so far as the different varieties of cloud form are concerned, and 

 it is only likely to be increased slowly and at long intervals by photo- 

 graphs of scarce forms of clouds or by particularly interesting series. 

 During the year the Secretary has secured many new negatives ; but 

 since the collection already includes satisfactory examples of the same 

 types, it has not been thought desirable to add more duplicates, and the 

 offers of co-operation from other photographers have not been fulfilled. 

 With regard to photographs of lightning also the collection has not been 

 increased, for your Committee have not been made aware of any such 

 photographs which show any features not already familiar, and no 

 opportunity has occurred for the Secretary to make any observations for 

 the further elucidation of the known phenomena. 



Your Committee propose to invite the Royal Meteorological Society 

 to take charge of such photographs from their collection as are not likely 

 to be required for further investigation. 



The attention of the Committee has been drawn to another application 

 of photography which seems to open up a possibility of very valuable 

 work ; this is in the measurement of cloud altitudes. This is a question 

 which has become more important since the acceptance by the Munich 



