2IO 



NA TURE 



[December 29, 1892 



his experiences in the north of Greenland. In March Prof. 

 Bonny will lecture on the action of ice in producing geogra- 

 phical forms, and there will be other papers dealing with the 

 scientific basis of geography. 



The death of Cardinal Lavigerie on November 24 removed 

 one of the most powerful personages who have recently influ- 

 enced the geography of Africa. It is very largely on account 

 of his labours that the French Roman Catholic missions have 

 played so conspicuous a part in combating the slave trade, and 

 to him also is due the formation of a much-needed Belgian 

 Anti slavery Society. 



The British Government having decided to relieve the 

 East African Company from the responsibility of occupying 

 Uganda, an Imperial commission, under the charge of Sir 

 Gerald Portal, will set out from Mombasa as soon as it 

 can be got ready to take over the administration of the country. 

 Another fact of some interest is the revival by Mr. Cecil Rhodes 

 of the idea of exploring Africa by telegraph. He proposes to 

 lay down a line from the Cape to Uganda, and uUimately to 

 extend it to Egypt. In a few months the South African Com- 

 pany's wires will have reached the mission station of Blantyre 

 north of the Zambesi, and there are no serious physical diffi- 

 culties in continuing the line to the head-waters of the Nile. 

 The effect on the exploration of Africa will be enormous, not 

 the least important result being the possibility of arriving at the 

 true longitudes of places in the interior of the continent. 



DEW AND FROST. 

 A PAMPHLET recording some interesting "Observations 

 -^ on Dew and Frost," by the Hon. R. Russell, has just been 

 published by Mr. Edward Stanford. We reprint Mr. Russell's 

 ' ' Summary of Results " : — 



The observations were begun with the object of verifying the 

 commonly received theory of dew, and with a strong feeling 

 ihat the results obtained by Col. Badgeley, described in the 

 Proceedi7igs of the Royal Meteorological Society for April, 1891, 

 opposed as they were in some measure to the accepted teaching 

 on the subject, would not be corroborated. When, after ex- 

 posing inverted glass tumblers and pans on grass and bare earth 

 in the summer of 189 1, dew was often found in surprising 

 amount in the interior, I attributed the deposit to vaporous air 

 which might have entered under the rim and parted with its 

 moisfare in the calm of the inclosed space. But when it was 

 found that a tumbler pressed down into dry earth, and other 

 vessels admitting little air from outside, were considerably be- 

 dewed in the interior ; and when, further, similar vessels inverted 

 on earthenware or metal plates were found to be very slightly or 

 not at all bedewed inside, it became more probable that the 

 vapour condensed in the interior of vessels over grass and garden 

 earth had proceeded from the earth beneath. Next, it was found 

 that china plates, admitting a flow of air between their lower 

 surfaces and the ground, were more heavily bedewed on their 

 lower than on their upper surfaces, and that a cylinder of glass 

 was most bedewed on the lower outer and upper inner surfaces. 

 These observations confirmed the suspicion that the dew on the 

 inside of the hollow vessels was derived from the ground. It was 

 for a long time a matter of doubt and difficulty that vessels 

 invened over dry, dusty earth and dry turf were found copiously 

 bedewed within on the morning following exposure. On many 

 mornings the amount of dew in the interior increased in some 

 proportion to the precautions taken to exclude free air, and it 

 seemed highly improbable that moist air penetrated, without de- 

 positing on its way much of its moisture, either through the 

 dusty earth banked round the edges of the vessel, and exposed 

 to the sky, or else through the dusty covering of earth below 

 the vessel from lower layers. 



In December, 1891, during hard frost and very fine weather, 

 with calm or very light airs, the ground being frozen hard, leaves 

 of bushes, ferns, &c., were seen 10 be frosted both on their upper 

 and lower sides, though much less on the lower sides facing the 

 bare ground than on the upper sides facing the open sky. Where 

 thick fern grew between the observed leaves and the ground, 

 there was no rime on the lower sides of the overhanging ferns or 

 leaves. This seemed to show that the rime on the lower sides 

 of ferns was due to exhalation from the ground, for the interruption 

 of radiant heat from ths earth by dry litter would rather favour 

 than reduce the frosting of the under sides. Live leaves on 

 bushes, and dead leaves on the ground, were whitened with 



NO. 1209. VOL. 47] 



frost on their upper sides, and had a thin film or coat of trans- 

 parent ice on their lower sides. Leaves and sticks on the 

 ground were less frosted on the sides facing the ground than on 

 the top. Thick planks between a few inches and one foot above 

 the ground were about a third as much frosted on the lower as 

 on ihe upper sides. Consideiing that the upper side of a plank 

 I inch thick would fall to a considerably lower temperature by 

 radiation than the lower side, it may be supposed that the de- 

 position would have been largest on the lower side if they had 

 bv en at the same temperature. That much frost came from the 

 air independently of the ground, was shown by the white roofs 

 12 feet above the surface of the earth. On the other hand the 

 grass was much more heavily frosted. Moreover, tumblers in- 

 verted and pressed down on dry, hard, bare earth, on sand, and 

 on hard turf, were moderately frosted inside, besides being thickly 

 frosted outside. The indications, on the whole, seemed to 

 resemble those of the previous June, but the vapour condens- 

 ation attributable to exhalation from the earth bore a much 

 smaller proportion to the total deposit than in the case of dew 

 on interior surfaces observed in summer 



Boards, tiles, and stones (sandstone) in heaps were frosted 

 on the top, and especially in cracks and indentations of the top 

 surface, but not in the interstices between the separate pieces. 

 Stones on the ground were sometimes not frosted at all on the 

 top, but much on the parts against the sandy earth, and where 

 bedded in the ground. 



Further experiments in May and in the summer months of 

 1892 gave strong confirmation of the evidence that much dew 

 and frost are caused by exhalation of vapour from the earth, even 

 in dry weather. 



The facts that— 



(i) A large quantity of dew was invariably found on clear 

 nights in the interior of closed vessels over grass and sand. 



(2) Very little or no dew was found in the interior of vessels 

 inverted over plates on the ground. 



(3) More dew was found on the lower side of a square, slightly 

 raised, china plate over grass or sand than on the lower side of a 

 similar plate placed upon the first. 



(4) The lower sides of stones, slates, and paper on grass or 

 sand, were much more dewed than the upper sides. The flat 

 wooden back of the minimum thermometer on clear evenings 

 when lying on earth, sand, or grass was almost invariably wet 

 before the upper surface. 



(5) The lower side of plates of glass, I or 2 in. above grass, 

 were as much or more bedewed than the upper sides. 



(6) Leaves of bushes, leaves lying on the ground, and blades 

 of grass were about equally bedewed on both sides. 



(7) The interior of closed vessels inverted on the grass and 

 covered with two other inverted vessels of badly-conducting 

 substance was thickly bedewed, and the grass in the three 

 circular inclosures also thickly bedewed. 



(8) The deposit of dew on the interior of closed vessels inverted 

 over dry garden earth was much less than over sand or turf, 

 although the powdery condition of the earth in the morning 

 showed that no deposit from the air had taken place on its sur- 

 face during the night. 



(9) Usually a greater amount of dew was deposited in the 

 interior of vessels when the earth was moist at a little depth 

 below the surface than when the earth was at its driest. 



(10) The temperature of the space under a glass plate or 

 other object suspended near the surface of the ground was higher 

 than that of the upper surface of the object, and, nevertheless, 

 a cloudy film was produced first on the lower surface, — amounted 

 to a proof that a large part of the dew formed is derived from 

 vapour from the earth. 



Moreover, the large diflference often observed between the 

 quantity of dew deposited in the interior of a vessel inclosing a 

 plant, and the quantity of an empty vessel, proved that much 

 dew may be derived from the earth through plants. 



Drinking glasses inverted over grassy tuif, and over turf close 

 by, from which the grass was removed, showed a similar excess 

 of deposit on the glasses inclosing grass. More vapour was 

 condensed on plates suspended over grass than over bare earth. 

 In these cases the conditions are somewhat artificial, and the 

 grass, which was covered by a suspended plate or inclosed by a 

 glass, would be warmer than if the exposure to the sky were free, 

 but the disturbance thus caused would tell as much against as 

 in favour of deposition on the interior surface. It may be 

 objected that the air in and above the grass would be colder, 

 owing to the radiating grass, than over the bared spot, and that 



