TRANSACTIONS OP THE SECTIONS. 49 



blew little more than half that time, or 4041 days. Does Orkney owe much of the 

 mildness of its climate to this prevalence of S. and W. winds ? 



Two other tables were given — one embracing a great variety of particulars regard- 

 ing Orkney, the other the same particulars regarding all the stations of the Scot- 

 tish Meteorological Society. From these it appears that the instruments do not show 

 more dampness in Orkney than in the other stations. 



Observations on the Natural Obstructions i?i the Atmosphere preventing the 

 view of Distant Objects on the Earth's Surface. By Alexander Cruick- 

 bhank, A.M., Aberdeen. 



I wished to determine the frequency with which the daily extreme limits of view 

 from any fixed station, over an extensive range of country, are circumscribed bv 

 natural causes, viz. haze, showers, and mist or low clouds. For this purpose I made 

 daily observations about noon, during the years 1S56, 1857, 1858, from the vicinity 

 of Aberdeen, on the distance seen along the South Deeside Grampian range of 

 mountains, which run in a S.W. direction from Aberdeen, and of which the main 

 tops are visible in fine weather from that vicinity, at the respective distances of five, 

 ten, twenty, thirty, forty, and fifty miles. These tops are called Clochendichter, 

 Caernmanearn, Caerloak, Mt. Battock, Mt. Keen, and Lochnagar, the last one being 

 nearly in the S.W. corner of Aberdeenshire. They are all nearly in a line, and were 

 taken as fixed points for noting the varying distances seen. It was found that the 

 obstructions to distant vision, caused by showers, and by mists or low clouds, entirely 

 obscured the view at once, at the neare'st points to the observer at which they existed. 

 When, however, haze circumscribed the view, it was found to increase gradually, 

 with the distance, beyond the nearest point at which it began to be perceptible, and 

 this was often only one or two miles off, till its density entirely prevented further 

 vision beyond the distance of five miles and upwards. 



Taking the average of the observations for the above-mentioned three years 1856, 

 1857, 1858, it was found that the view, about noon, extended to fifty miles, or to 

 Lochnagar, on ninety-four days during the year; but the state of the atmospheric 

 obstructions to vision at that distance showed that, on many of those days, the view 

 must have reached much further, had the observer been at a higher level. On fifty- 

 two days the view was limited to forty miles, or to Mt. Keen, the nearer hills being 

 also seen, or it was less than fifty miles ; Lochnagar, though seen in clearer weather 

 from the point of observation, being rendered invisible by the atmospheric obscurities 

 referred to. On forty-five, fifty-one, thirty-nine, and sixty-nine days during the year, 

 the view at the time of observation was respectively limited to the distance of thirty 

 miles, or Mt. Battock, twenty miles, or Caerloak, ten miles, or Caernmanearn, and 

 five miles, or Clochendichter, more distant vision being completely prevented by the 

 atmospheric obscurities beyond those distances. In fine, on fifteen days in the year, 

 mist and showers circumscribed the view within one mile of the place of obser- 

 vation. 



Simultaneous observations in other directions from the point of observation, gave 

 similar results, in as far as the inequalities of the earth's surface permitted a suffi- 

 cient length of radial view. Had, therefore, the view about noon over the earth's 

 surface extended as far in all directions as in that of Deeside, there would have 

 been visible on an average of 94, 52, 45, 51, 39, 69, and 15 days in the year, portions 

 of that surface included within circles of the diameters of 100, 80, 60, 40, 20, 10, 

 and 2 miles respectively. 



From the similar variability of climate throughout Britain, the above results may 

 be regarded as true, or nearly so, of the rest of the island. They show that angles sub- 

 tended by objects at the distance of fifty miles or more, such as enter into the Ordnance 

 Survey of the country, could not, on account of haze, mist, &c, be observed with 

 sufficient accuracy about noon on more than ninety-four days in the year. There is, 

 however, at that time of the day another very frequent and great obstruction to such 

 observations, viz. the tremor of the atmosphere, on the frequency and amount of 

 which the present observer does not feel qualified to report. But it is well known 

 that, owing to the two causes now mentioned — obscurity and unsteadiness of vision — 

 the Ordnance surveyors, with the whole day at their command, have sometimes been 



1859. 4. 



