258 



NA TURE 



[Jan. 14, I i 



already known, wind hinders the formation of dew by preventing 

 an accumulation of moist air near the surface of the ground. 



An examination of the different forms of vegetation w as made 

 on dewy nights. It was soon evident that something else than 

 radiation and condensation was at work to produce the varied 

 appearances then seen on plants. Some kinds of plants were 

 found to be wet, while others of a different kind, and growing 

 close to them, were dry, and even on the same plant some 

 branches were wet, whilst others were dry. The examination of 

 the leaf of a broccoli plant showed better than any other that 

 the wetting was not what we might expect if it were dew. The 

 surface of the leaf was not wet all over, and the amount of de- 

 posit on any part had no relation to its exposure to radiation, or 

 access to moist air ; but the moisture was collected in little 

 drops, placed at short distances apart, along the very edje of 

 the leaf. Closer examination showed that the position of tliese 

 drops had a close relation to the structure of the leaf; they were 

 all placed at the points wh.;re the veins in the leaf came to the 

 outer edge, at once suggesting that these veins were the channels 

 through which the liquid had been expelled. An examination of 

 grass revealed a similar condition of matters : the moisture was 

 not equally distributed over the blade, but was in drops attached 

 to the tips of some of the blades. These drops, seen on vegeta- 

 tion on dewy nights, are therefore not dew at all, but are an 

 effect of the vitality of the p'.ant. 



It is pointed out that the excretion of drops of liquid by plants 

 is no new discovery, as it has been long well known, and the 

 experiments of Dr. Moll on this subject are referred to ; but 

 what seems strange is that the relation of it to dew does not 

 seem to have been recognised. 



Some experiments were made on this subject in its relation to 

 dew. Leaves of plants that had been seen to be wet on dewy 

 nights were experimented on. They were connected by means 

 of an india-rubber tube with a head of water of about I metre, 

 and the leaf surrounded with saturated air. All were found to 

 exude a watery liquid after being subjected to pressure for some 

 hours, and a broccoli lexf got studded all along its edge with 

 drops, and presented exactly the same appearance it did on 

 dewy nights. A stem of grass was also found to exude at the 

 tips of one or two bl.ides when pressure was applied. 



The question as to whether these drops are really exuded by 

 the plant, or are produced in soaie other way, is considered. The 

 tip of a blade of grass was put under conditions in which it could 

 not extract moisture from tlie surrounding air, and, as the drop 

 grew as rapidly under these conditions as did those on the un- 

 protected blades, it is concluded that these drops are really 

 exuded by the plant. Grass was found to get "dewed" in air 

 not quite saturated. 



On many nights no true dew is formed, and nothing but these 

 exuded drops appear on the grass ; and on all nights when 

 vegetation is active, these drops appear before the true dew, and 

 if the radiation is strong enough and the supply of vapour suffi- 

 cient, true dew makes its appearance, and now the plants 

 get equally wet all over, in the same manner as dead matter. 

 The difference between true dew on grass, and these e.xudtd 

 drops, can be detected at a glance. The drops are always 

 exuded at a point near the tip of the blade, and form a drop of 

 some size, while true dew is distributed all over the blade. The 

 exuded liquid forms a large diamond-like drop, while the dew 

 coats the blade with a pearly lustre. 



Towards the end of the paper the radiating powers of different 

 surfaces at night is considered, and after a reference to some 

 early experiments on this subject, the pa[ier proceeds to describe 

 some experiments madewith the radiation-thermometer described 

 by the author in a previous paper. When working with this 

 instrument it is placed in a situation having a clear view of the 

 sky all round, and is fixed at the same height as the ordinary 

 thermometer-screen, which is worked along with it, the differ- 

 ence between the thermometer in the screen and the radiation- 

 thermometer being observed. This difference in clear nights 

 amounts to from 7' to 10°. By means of the radiation-thermo- 

 meter the radiating powers of diflerent surfaces were observed. 

 Black and white cloths were found to radiate equally well ; soil 

 and grass were also almost exactly equal to each other. Lamp- 

 black was equal to whitening. Sulphur was about 2/3rds of 

 black paint, and polished tin about i/7th of blrck paint. Snow 

 in the shade on a bright day was at midday 7^ colder than the 

 air, while a black surface at the same time was only 4° colder. 

 This difference diminished as the sun got lower, and at night 

 both radiated ahnost equally well. In the concluding pages of 

 the paper some less important subjects are considered. 



TELESCOPIC SEARCH FOR THE TRANS- 

 NEPTUNIAN PLANET'^ 

 T N the twentieth volume of the Ameiican Journal of Science, at 

 page 225, I gave a preliminary account of my search, theo- 

 retic and practical, for the trans-Neptunian planet. I say the 

 tr.ans-Neptunian planet, because I regard the evidence of its 

 existence as well-founded, and further, because, since the time 

 when I was eng.ajJed upon this search, nothing has in the least 

 weakened my entire conviction as to its existence in about that 

 part of the sky assigned ; while, as is well known, the inde- 

 pendent researches in cometary perturbations by Prof Forbes 

 conducted him to a result identical with my own, — a coincidence 

 not to be lightly set aside as pure accident. 



That five years have elapsed since this coincidence was re- 

 marked, and the planet is still unfound, is not sufficient assur- 

 ance to me that its existence is merely fanciful. In so far as I 

 am informed, this spot of the sky has received very little 

 scrutiny with telescopes competent to such a search ; and most 

 observers finding nothing would, I suspect, prefer not to 

 announce their ineffective search. 



The time has now come when this search can be profitably 

 undertaken by any observer having the rare combination of 

 time, enthusiasm, and the necessary appliances. Strongly 

 marked developments in astronomical photography have been 

 effected since this optical search was conducted ; and the 

 capacity of the modern dry-plate for the registry of the light of 

 very faint stars makes the application of this method the shortest 

 and surest way of detecting any such object. Nor is this purely 

 an opinion of my own. But the required apparatus would be 

 costly ; and the instrument, together with tire services of an 

 astronomer and a photographer, woidd, for the time being, be 

 necessarily devoted exclusively to the work. While, however, 

 the photographic search might have to be ended with a negative 

 result, in so far as ihe trans-Neptunian planet is concerned, there 

 would still reaiain the series of photographic maps of the region 

 explored, and these would be of incalculable service in the 

 astronomy of the future. 



In the latter part of the paper alluded to above, I stated the 

 speculative basis upon which I restricted the stellar region to be 

 examined ; also the fact that between November of 1S77 and 

 March of 1S7S I was engaged in a telescopic scrutiny of this 

 region, employing the twenty-six-inch refractor of the Naval 

 Observatory. For the purposes contemplated I had no hesitation 

 in adopting the method of search whereby I expected to detect 

 the planet by the contrast of its disk and light with the appear- 

 ance of an average star of about the thirteenth magnitude. A 

 power of 600 diameters was often e.nployed, but the field of 

 view of this eyepiece was so restricted that a power of 400 

 diameters had to be used most of tlie time. 1 say, too, that, 

 "after the first few nights, I was surpri-cd at the readiness with 

 which my eye detected any variation from the average appear- 

 ance of a star of a given faint magnitude : as a consequence 

 whereof my observing-book contains a large stock of memoranda 

 of suspected objects. JMy general plan with these was to observe 

 with a sufficient degree of accuracy the position of all suspected 

 objects. On the succeeding night of observation they were re- 

 observed ; and, at an interval of several weeks thereafter, the 

 observation was again verified." Suujoined to the original 

 observations are printed these verifications in heavy-faced type. 



In conducting the search, the plans were several times varied 

 in slight detail, — generally because experience with the work 

 enabled me to make improvements in method. Usually I 

 prepared every few days a new zone-chart the region of over 

 which I was about to search ; and these charts, while containing 

 memoranda of all the instrumental data which could l)e pre- 

 pared beforehand, were likewise so adjusted with reference to 

 the opposition-time of the planet as to avoid, if possible, its 

 stationary point. The same thing, too, was kept in mind in 

 selecting the times of sub equent observation. Notwithstanding 

 this precaution, however, it would be well if some observer who 

 has a large telescope should now re-examine the positions of 

 these objects. 



Researches in faint nebulae and nebulous stars appearing 

 likely to constitute a separate and interesting branch of the 

 astronomy of the future, it has seemed to me that the astro- 

 nomers engaged in this work may like to make a careful exami- 

 nation of some of the stars entered in my observing-book under 

 the category of "suspected objects" The method 1 adopted of 



' By David P. Todd, M.A., from the PnceeJings of lh= American 

 Academy of Arts and Sciences. 



