Nov. I, i8S6.] 



THE TROPICAL AGRICULTURIST. 



uy 



THE NATURE OF DEW. 



The researches of Dr. Wells into the nature of dew, 

 made a great many years ago, are still acknowledged 

 as disclosing the true theory of the phenomenon, and 

 yet there is ample room for further investigation. 

 The popular idea is that dew falls, and anxious mothers 

 still caution their children, especially if they are delicate 

 in health, against exposing themselves to the falling 

 dew. Dr. Wells proved that dew did not come either 

 from the earth below nor from the sky above, but 

 that it was condensed out of the air in contact with 

 plants and other cold surfaces. And yet the popular idea 

 not erroneous, for the vapor in the air may be 

 condensed on floating motes and du.st, giving 

 rise to what may fairly enough be denominated fall- 

 ing dew, although it is really a gentle rain formed 

 not far from the surface of the earth. Whether it 

 should be called dew or invisible rain is a mere 

 matter of taste, and a question of definition. There 

 is really no reason why the popular phraseology should 

 be changed, and even from a scientific point of view 

 the term " falling dew " may be defended as prefer- 

 able to "invisible rain." Dui'ing a fog the beards 

 and whiskers of men frequently collect quite a large 

 quantity of dew, while in very cold regions the 

 eyebrows may collect dew which is converted into 

 hoar-frost. The fact is that dew is deposited when- 

 ever the grass, the air, or the hair falls in temperature 

 below the dew-point. The analogy may be carried 

 further, and it may properly be asked whether fog is 

 anything else than floating dew. If the answer be 

 in the affirmative, then the whiskers may be supposed 

 to collect the dew already existing in the air, and 

 the difference between falling dew and fog may merely 

 be one of mass or quantity. Dr. A\"ells was no doubt 

 quite right in saying that the dew which is vi.sible 

 on the surface of leaves, is condensed upon them 

 directly from contact with moist air and yet it is 

 quite possible that the quantity is increased by the 

 incidence of falling dew— the phenomenon thus being 

 the result of two or more influences. Prior to his 

 time, there had been much discussion as to whether 

 the dew rose from the earth below, or fell trom the 

 sky above. It had been ascertained that there was 

 a heavier deposit of dew inside a bell-jar than there 

 was outside, and it was argued that the dew inside 

 must have ascended from the earth. The correct 

 inference is that the vapour already pre-existent in 

 the air is reinforced by an accession of vapor from 

 the earth, the heat absorbed during the day serving 

 to maintain evaporation after sunset, while the bell- 

 glass merely served to prevent the dissipation of this 

 vapor by the wind. 



Some further light has been thrown by Mr. Aitken, 

 of Edinburgh, on the dew question. He asked himself 

 why dew was not deposited on gravel paths and dry 

 ground, as well as on the vegetation in adjoining 

 beds, and he has found the answer, which is— that 

 dew is deposited on the gravel and dry earth, but it 

 has to be looked for in the right place, namely, on 

 the under-side of pebbles, while some of it is no doubt 

 absorbed. There should, however, I imagine, be less 

 dew on dry patches, because there is a more free 

 circulation of air by which the vapor is carried off. 

 The vegetation itself imprisons the moist air, and thus 

 favors the deposition of dew. 



Another form of dew, or of what is taken for dew, 

 was discovered by Mr. . Aitken. He found that some 

 plants were more bedewed than others, that the sur- 

 face of the leaf was not wet all over, and that the 

 deposit on any part had no relation to its exposure 

 to radiation or access to moist air. The position of 

 the drops of apparent dew had a close relation to 

 the structure of the leaf. On brocoli plituts the 

 drops were all placed at the points where the veins 

 of the leaf came to the outer edge. On grass the 

 moisture was in drops attached to the tips of some 

 of the blades. Other considerations led him to the 

 behef that these drops were not dew at all, but 

 exudations from the plants. That plants exude such 

 globules has been pointed out by other observers, 

 but Mr. Aitken is the first to point out their relation 

 to the appearance of dew. The explanation is exceed- 



ingly probable. The exudation is no doubt going on 

 in all weathers, but it is only on dewy nights that 

 the drops will hang to the plant. When the air is 

 dry they are likely to be carried away as vapor. 

 Experiments were made on leaves protected from all 

 contact with the atmosphere, and on these the 

 exudations made their appearance. These facts do 

 not in any way invalidate the accepted theory of dew, 

 but they serve to show that the phenomena are more 

 complicated than has hitherto been supposed, 



Mr. Aitken also made experiments on the radiatioa 

 by night of several substances, and lie found that 

 they gave very difl:erent results from those obtained 

 by experiments mafe in the day time. Black and 

 white cloths were found to radiate equally well, a 

 result which invalidates the received theory that 

 absorption and radi.ition are equivalent. Franklin's 

 experiment on the melting of snow under patches of 

 black and white cloth in full sunshine will be here 

 called to mind. Soil and grass radiate equally well 

 by night. Lampblack and whitening are also alike. 

 Snow in the shade on a bright day was 7 degrees 

 colder than the air, while a black surface was only 4 

 degrees colder. This difference diminished as the sun 

 got lower, and at night both radiated equally well. 

 These facts are exceedingly instructive, — header. 



ENEMIES OF THE SUGAR-CANE, 



In India little or nothing has yet been done iu 

 the way of protecting its agriculturists from the losses 

 caused by insects. There is a school of forestry at 

 Dehra, and the instruction given there includes some 

 accouut of the insects useful or hurtful to man and 

 his industries, and we see references as to individual 

 insects from time to time made to Mr. Wood-Mason. 

 Some years ay;o Mr. Thompson, of the Forest Depart- 

 ment, contrit)uted a valuable pamphlet on the enemies 

 of the forest trees; Mr. Haldane's " All about Grubs " 

 mentions several beetles which injure the coffee shrubs ; 

 antl Mr. Nietner in Ceylon, and Dr. Bidie of Madras, 

 have added considerably to the existing information 

 regarding them. But India has no reports or compen- 

 diums .such as iu England periodically issue from 

 Miss Ormerod's pen, and the first attempt to furnish 

 a general view of the Indian agriculturists' insect 

 foes has been given in the " Oyclop.'elia of India," 

 the third feditiou of which was mentioned in our 

 issue of .Tuly 6. Yet the tillers of the soil of India 

 are skilful, hard-working husbandmen and gardeners, 

 but they are great sufferers from blights, and their 

 patient toil should win for them all the care which 

 the scientific skill at the command of Clovernment 

 can bestow. They are in many tribes and of ditt'eront 

 races, the farmers of recognised superiority being the 

 Oha^a and Khisan of Bengal, the Kuubi and Kurinil 

 of the Western and Northern Provinces, the Tamil 

 Vellalar, the Teling Keddi, Kapa, and Kama, and 

 the 'Janarese Wahala, the finer garden work being- 

 carried on by the Tota-Kara, the Mali, the Kaeh hi, 

 the. Lodha, and others. We have been led to these 

 remarks by the perusal of a pamphlet on, "The 

 Animal Parasites of the Sugar-cane," by Hy-Ling Itoth 

 (i'riibuer & Co.). The cane is to be seen growing 

 everywhere throughout British India, and, although 

 used by the people more as a fruit than for sugar- 

 making, it takes a prominent place among their garden 

 plants. It receives from them great care for it is 

 a costly plant to grow, occupying the grounds for 

 many munths, requiring a rich soil, with plciiby of 

 manure and abundance of water. It is lialjle to be 

 attacked by several iiisect^f, ;ind if, from boisterous 

 wimis, the tops become twisted, the growth is cheeked 

 and the cane rots. The planters of the W'e.-t luditis. 

 South America, the jMauritius and Aiistraliii, have 

 given much attention to the investigation of the 

 diseases au.l injuries of the cane, aii'i Mr. Pioth h.is 

 dune good service to the planting industry and to 

 science by summarising all that is known of its auiiiial 

 parasites. An examination of growing cane exhibits 

 a variety of animal lite which is simply marvellous ; 

 and the planter who thinks Iktle of a single parasite 



