KovtiMfiEfi 2, 1891.] 



KNOWLEDGE. 



215 



and others have for tlie past seven years made very careful 

 observations during the whole year. 



t)r. Alexander Buchan, the secretary of the Scottish 

 MetBorolbgical Society, has recently published in the 

 .Journal of the Society an outline of the observations made 

 at the Ben Nevis Observatory and at Fort William during 

 tliat period, and again in the " Transactions of the 

 Royal Society of Edinburgh." From this account we find 

 that for that period the mean barometric pressure at tile 

 height of 4407 feet (tlie top of the Ben) is 25-294 inches, 

 and that the mean barometric pressure at sea level 

 (Fort William) is 29-856 inches, the difference being 4-562 

 inches, or about an inch to the 1000 feet of difference of 

 level. The difference reaches the maximum of 4-041 inches 

 in February, and the minimum of 4-484 in July. 



The mean temperatures at the top and bottom are 30-9° 

 and 46-8' Fahr. respectively, the difference for 4407 feet 

 of altitude being 15-9°. The difference reaches the maxi- 

 mum of 18'5° in April, and the minimum of 10-9° in 

 January. The absolutely highest temperature at the Ob- 

 servatory was 070'', on June 24th, 1887, and the lowest, 

 6-4°, on February 10th, 1889. 



The mean annual rainfall at the Observatory has been 

 140-17 inches; and at the base 78-19 inches ; or 01-98 

 inches less at the base than at the top of the Ben. 



The direction of the winds on the ]>en indicates a well- 

 marked diurnal variation. From 8 a.m. to 8 a.m. 

 northerly winds (strength 2| Iniles an hour) prevail, 

 whereas from 11 a.m. to 2 p.m., southerly winds (a little 

 stronger) are in the ascendant. The coldest wind is from 

 the north-east and the warmest from the south. 



The electrical phenomenon, called St. Elmo's Fire, is 

 often observed on the Ben, especially during the winter 

 months. The weather which precedes, accompanies, and 

 follows this phenomenon has quite marlved characteristics. 

 Stormy weather is called at the Observatory " St. Elmo's 

 weather," so peculiar is it. In almost every case another 

 cyclone, with its spell of bad weather, follows the particular 

 cyclone in which St. Elmo's Fire is observed. Seventy per 

 cent, of the thunderstorms occur from September to 

 February, being very rare in summer — the very reverse of 

 what occurs in the east of Scotland. But at Fort William 

 summer thunderstorms are twice as frequent as at the 

 Observatory, suggesting that a considerable number must 

 be below the summit. The winter thunderstorms prevail 

 at night ; the reverse is the case in summer. 



Professor C. Miohie Smith has shown that, on the edge 

 of a dissolving mist, the potential is lower than the 

 average, but higher on the edge of a condensing mist. 

 This is corroborated by the observations on Ben Nevis. 

 When the, top of the Ben becomes clear for a sliort time, a 

 strong current comes up the telegraph wire from the base 

 to the summit. But as soon as the summit is again mist- 

 clad, the current is reversed. During a fall of rain the 

 cm-rent nearly always passes down the wire ; and in a 

 sudden shower this current is very strong. When the 

 rain stops the current passes upwards again. 



Some very curious results are given about the enumera- 

 tion of the dust-particles of the air, by means of the 

 ingenious apparatus invented by Mr. Jolm Aitlien, of 

 Falldrk. This apparatus we described in the October 

 number of Knowledge last year. On 31st March of last 

 year, at 4.30 p.m., the summit of the Bou was clear, and 

 the number of dust-particles per cubic inch was 46,400, but 

 shortly afterwards a thickness was observed approaching 

 from the south-west, which by 6 p.m. reached the Obser- 

 vatory, and the dust-particles rose to 214,400 per cubi« 

 inch — the maximum observed since. On June 15tlithe 

 number fell from 15,600 at midnight to 840 at 10.30 a.m. 



But the observations on the 20th July were exceptionally 

 remarkable. At Fort William the theiinometer remained 

 constant at 35'' from 9 p.m. till 4 a.m. next day. But at 

 the top there was a most marked variation of temperature. 

 At 10 p.m. the wind suddenly veered fi-om south-west to 

 north, increasing to 40 miles an hour, and the temperature 

 rose from 41° to 47°, and soon after to 49-2°. Ten 

 observations were made with Aitken's dust-enumerator, 

 between 2 and 3 a.m., and the extraordinarily low mean 

 of Only 34 dust-particles in the cubic inch was registered. 

 From our last notice it will be found that 3500 per cubic 

 inch is the lowest figure ascertained by Mr. Aitken, and 

 that was in Switzerland. The peculiarity of this minimum 

 register on the Ben is thus accounted for. A warm 

 highly-saturated north wind was blowing out of the 

 cyclone, which lay to the northward ; whilst the sea-level 

 wind, which was south-west, was blowing in upon the 

 same cyclone. It is evident, then, that there is an intimate 

 relationship between the number of dust-particles and the 

 cyclones and anti-cyclones over North -Westei-n Europe at 

 the same time. 



Halos, coronw, fog-bows, glories, and other optical 

 phenomena hold a prominent place among the observations 

 on the Ben. But the most laborious investigation has 

 been the determination of the rate of diminution of 

 temperature with heiglit, and the rate of the diminution 

 of pressure for the different air-temperatures and sea-level 

 pressures that occur. 



BIRDS AND BERRIES. 



By the Rev. Alex. S. Wilson, M.A., B.Sc. 

 (Continued from par/e 182.) 



THERE are a few cases in which fruits though not 

 succulent appear to depend on birds for the 

 dispersion of their seeds. The snake-nut of 

 Demerara [Ophioconjon pntadoxum) is so called 

 on account of its peculiar coiled embryo pre- 

 senting a striking resemblance to a small snake. The 

 Hkeness is so marked that when the egg-like capsule 

 is opened, one involuntarily starts back fi-om the supposed 

 reptile. It would seem that the imitation here is intended 

 as a deception ; for if a bird seizes the seed imder 

 the impression that it has got a snake, and after carrying 

 it some distance discovers its mistake and lets it drop, 

 then the object of the plant— dissemination — has been 

 attained, and that without any outlay m the shape of 

 succulent pulp or saccharine matter. In like manner the 

 pod of Sciirjiiunts suhnllosti has a curious resemblance to a 

 centipede, and that of 8. irniiii'ulKtd to a worm or cater- 

 pillar. The long hanging pods of Triihoxantlus amjuimi, as 

 its name indicates, look very much like snakes. Iii.'<i-rntla 

 pcU'chius resembles a centipede. According to Lubbock, 

 the seeds of Ahrux pirciitiiritis, Mdityuid (tiamlrn, JnlroiiJui 

 and Uiviims mimic beetles, while several lupines have 

 seeds resembling spiders. 



The advantage of the mimicry in the rosary beau 

 (Abnis) is easily understood. The beans are bright 

 scarlet with a black, glossy patch. When the pod 

 dehisces they arc exposed to view and attract, we shall 

 suppose, an insectivorous bird which mistakes them for a 

 particular kind of beetle. After carrying the bean some 

 distance the bird discovers its error, drops the seed, and 

 thus gratuitously accomplishes the dissemination of Abntx. 

 The seeds of ClinHlnulron also appear to be mimetic, and 

 remind one somewhat of the rosary bean. I'ossibly this 

 explanation also applies to the common cow-wheat, the 



