METEOROLOGY OF BEN NEVIS. xlix 



the real earth, or zero of potential, it is obvious that these results are confirmatory 

 of theory advanced by Professor Michie Smith, a conclusive proof of which would 

 be of great importance in investigations connected with thunderstorms.* 



Optical Phenomena. — The resolution to erect an Observatory on Ben Nevis at 

 once called attention to the fact that many branches of science, besides the ordinary 

 meteorological ones, might obtain from the. Observers data of great value, not easily 

 to be obtained elsewhere. Among these the very interesting Optical Phenomena 

 exhibited in sunshine by mountain mists hold a prominent place. Coronse and Fog- 

 bows are common enough appearances at low stations, but Glories are very rare and 

 always extremely imperfect, — so much so, in fact, that the theory of their production 

 is not yet certain. It became, therefore, extremely desirable that the Ben Nevis 

 Observers should be instructed to look out for such appearances, and should be 

 furnished with some simple means of measuring them with sufficient accuracy. As 

 the measurements required are merely angular diameters of coloured rings, a very 

 rudimentary apparatus amply suffices for the purpose ; and its indications, along 

 with eye observations of the distribution of colours in the various rings, contain all 

 that need be observed. Mr Omond has already communicated to the Royal Society 

 of Edinburgh a very extensive series of measures of Glories,t from which, no doubt, 

 materials for the testing of the various theories of their origin may be obtained 

 (see Plate). As a check on these measurements, he has given a number of details of 

 dimensions of Coronse, Rainbows, and Halos. The latter, especially, give an excellent 

 test of accuracy of angular measure. 



Relations of the Observations to Weather. 



This important application of the observations to practical matters has been 

 in progress for some time, and, so far as the investigation has gone, it is clear 

 that considerable and especially rapid changes from the normal differences 

 of temperature and pressure are frequent concomitants and precursors of 

 storms, but more particularly of destructive winds during storms. Of these 

 departures from the normals the most important are the cases that show 

 an unusually rapid decrease of temperature with height — a result which is 

 only what was to have been expected, seeing that such observations indicate 

 a disturbance of the equilibrium of the atmosphere. It is well known that the 

 most destructive winds that accompany tornados occur where they are preceded 

 by a highly super-heated and super-saturated atmosphere near the earth's sur- 

 face, and are followed by markedly colder and drier weather. These and 



* "Observations on Earth-Currents in Ben Nevis Observatory Telegraph Cable, by H. N. Dickson, 

 Proc. Roy. Soc. Edin., vol. xiii. p. 530. 



+ " Glories, Halos, and Corona? seen from Ben Nevis Observatory," by R. T. Omond, First Paper, 

 Proc. Boy. Soc. Edin., vol. xiii. p. 500 ; Second Paper, vol. xiv. p. 314. 



vol. xxxiv. g 



