530 MR A. J. HERBERTSON ON 



time. The results showed that Glaisher's tables extended by using his factors were 

 unsatisfactory, for, to quote Dr Buchan, ' As regards the remarkably dry states of the 

 air, which form so prominent a feature in the climatology of Ben Nevis, Glaisher's 

 factors were found to be altogether inapplicable, and for such hygrometric observations 

 a specially constructed set of tables is therefore required.' * Comparing his dew-point 

 results with the values given in Glaisher's tables, Dr Dickson found t that the 

 tabulated dew-points ' are at first lower than those found by the hygrometer, the 

 difference first increasing to a maximum, and then diminishing to zero. After the 

 point of coincidence, the calculated dew-points are above those got directly, and the 

 differences go on increasing rapidly.' Leaving out of account the extreme cases, 'it is 

 to be remarked that the results deduced from Glaisher's tables ... do not, on the 

 whole, agree so well with direct observations at high levels as with those low down.' 

 One result of these investigations of Dr Dickson's is to show that if the ordinary 

 employment of these tables does not involve any considerable error, this is not true for 

 extensions of them based on the same factors. 



The data for calculating the special hygrometrical tables to be used for Ben Nevis 

 observations during very dry weather have not yet been obtained, and the reductions 

 of dew-points, vapour pressures, and relative humidities are at present made by 

 means of an extension of Glaisher's tables. This is eminently unsatisfactory, but it 

 must be observed that if one system of making reductions is used for very dry states of 

 the air, the same system ought to be employed in ordinary conditions. 



The Directors of Ben Nevis Observatory, and the members of the Ben Nevis 

 Committee of the British Association, have stated in their reports the need for arriving 

 ' at some accurate knowledge of the absolute amounts of aqueous vapour at different 

 heights under different weather conditions,' and suggested ' the desirableness of in- 

 stituting a system of special observations on the hygrometry of the atmosphere at the 

 two observatories, with a third station half-way up the hill.' J 



The nomination of the writer in 1892 by the University of Edinburgh to a research 

 scholarship, granted by the Commissioners for the Exhibition of 1851, gave him the 

 opportunity for continuing hygrometrical research, and to co-operate with the Directors 

 of the Ben Nevis Observatory. 



September 1892 to January 1893 was spent at Ben Nevis mainly at the High 



Level Observatory. Owing to the limited accommodation the work of an ordinary observer 



had also to be undertaken, but every fine moment was used for hygrometric work. In 



the spring of 1893 some time was spent in Professor Friedel's Laboratory in Paris 



testing the methods used during the previous winter, which were found to be satisfactory. 



Unfortunately a medical prohibition prevented further high level work, and in August 



and September 1893 the writer carried out the experiments at Fort- William, while 



at Ben Nevis Messrs A. Rankin, F. J. Hambly, and T. R Marr were good enough to 



* Op. n't., p. xlviii. t Proceeding* R.S.E., vol. xiii. p. 958. 



t Report of Council to Meeting of S.M.S., 7th March 1892. 



