420 



NATURE 



[December 17, 1914 



volunteers in town and country districts, wlpo would 

 take daily readings with the Ka'ta thermometer, accord- 

 ing to directions accompanying each instrument, which 

 we would supply. The time for taking an observation 

 need not be more than five or six minutes. . Full de- 

 tails of the Kata thermometer, its construction and . 

 use appear in Dr. Hill's Report of \'entilation recentlv 

 published by the Local Government Board. Jf any 

 of your readers are interested enough in the iiiatter 

 to take these readings we should be glad if thev 

 would communicate with us. 



Leonard Hill. 



ALartin Flack. 



O. W. Griffith. 

 London Hospital Medical College, 

 London, E. 



December 7. 



Forests and Floods. 



L\ the issue of July 16 Nature published a letter on 

 the above subject in which I showed that pulverised 

 soil holds more of the rainfall than the same soil 

 after it is consolidated by time and rainfall, and it was 

 pointed out that trees bv the growth of their roots 

 prevented the consolidation of the soil, and so enabled 

 forest lands to retain more of the rainfall than un- 

 disturbed ground. 



In that letter it was mentioned water passed more 

 quickly through consolidated soil than through the 

 same soil after it is pulverised. I have recentlv had 

 an opportunity of observing how this peculiarity of 

 dry soil acts in its disposal of the rainfall. In 'this' 

 district we had a very dry autumn ; there was only 

 0-43 of an inch of rainfall in the 37 davs extending 

 from September 18 to October 24. The soil in the 

 part of the garden to be referred to is of a light 

 and sandy nature, and became very dry, though the 

 plants seemed to get enough moisture' for their re- 

 quirements. The soil at the surface and to a depth 

 of at least one foot seemed to be very dry. Before 

 the end of the drought there were sorne cold nights, 

 and the surface began to look more moist; this would, 

 probably be caused by the condensation of the rising 

 vapour in the cold surface soil. When the rain began 

 on October 25 the surface was thus in a good con- 

 dition for absorbing it, and as the rainfall was never 

 at any time heavy, and was well distributed over the 

 eight days, during which 1-5 inches fell, none of it 

 would run off the surface, as even during very rapid 

 rainfalls the water easily enters that light soil. A 

 day or two after the rain had ceased I was putting 

 in some bulbs in the border, and was much surprised to 

 find that below a depth of a little more than 3 inches 

 of damp soil all underneath was as dry as it was 

 before the rain began; while I find from tests made 

 with soil lifted from the border, dried and broken 

 up, that 1-5 inches of water wetted it to a depth of 

 6 inches. It was shown in the previous letter that 

 pulverised soil holds a good deal more water than 

 thoroughly wetted consolidated soil, so that the 

 border was only moistened by the rainfall to one half 

 the depth it would have been if all the rainfall had 

 been retained by the upper layer of soil. The water 

 evidently had not gone uniformly through the soil, 

 but had made drainage tubes at certain places through 

 which it had passed underground, leaving parts of the 

 soil dry. This we might expect to happen if the soil 

 was not easily wetted, or there were certain parts 

 where. the soil was more prepared to take water than 

 at pthers. Such parts we might suppose to be those 

 up which the water vapour rose most freelv, and 

 would therefore be damper, or the water mav have 

 escaped down the outsides of the roots of the plants. 



NO. 2355, VOL. 94] 



Now in well stirred soil there are none of these pre- 

 ferential routes for the escape of the water. 



In illustration of these preferential routes one might 

 point to the condition of the soil on vertical cuttings 

 which one sometimes sees where roads are cut through 

 banks, or on any deep cutting made in soil. It will 

 be noticed that these cuttings have no vegetation on 

 them, and that the soil gradually weathers away, 

 leaving an overhanging turf. It will be noticed that 

 the soil on the face of the bank under the 

 turf remains dry, all the summer at least, how- 

 ever heavy the rainfall. The water evidently 

 does not percolate straight to the soil underneath, but 

 takes a preferential route, backwards from the turf, 

 and downwards behind the dry soil. The dry soil 

 seems to reject the water, which prefers to pass 

 through the damp soil behind it. It was noticed at 

 the end of November this year that these banks were 

 still dry though 64 inches of rain had fallen since 

 October 25. 



Some soils seem to have very little affinity for water, 

 and act very much in the same manner as most sub- 

 stances do towards mercury. For instance, when one 

 watches the seashore on a calm sunny day while the 

 tide is rising one often sees all Icinds of dry particles 

 which are much heavier than water floating on the 

 surface ; these particles may be sand or other earthy 

 matter. Again, when walking over bare sandy soil 

 one sometimes sees a curious illustration of this ob- 

 jection of dry sand to being wetted. If the weather 

 has previously been very drying and there comes a 

 shower of rain, some of the little hollows in the 

 sand will get full of water, and retaining it as well 

 as if they were made of puddle clay. Of course these 

 cups are very shallow, otherwise the hydrostatic pres- 

 sure would be sufficient to break the water film 

 between the grains of sand. 



However, repulsion of the water by ordinary soils 

 does not seem to explain the passage of the rainfall 

 through the soil without wetting it equally, A small 

 clod of soil from the border referred to was dried, 

 after which it was placed in a shallow vessel in which 

 was less than i mm. of water. The clod quickly 

 absorbed the water, raising it to its top surface, which 

 was fully I cm. above the level of the water. 



It is difticult to explain this preference of the water 

 for certain parts of the soil, where the soil wets so 

 easily on contact with it. Is it possible that the water 

 by sinking a little deeper at certain places comes to 

 exert a kind of negative hydrostatic pressure, and so 

 draw the water from the surrounding area? This, 

 if once started, would have a gradually increasing 

 influence owing to the increasing "head." 



The border referred to was opened up at a number 

 of places on November 23, after nearly 4 inches of 

 rain had fallen, and even then there were parts of the 

 soil quite dry- a few inches below the surface. It is 

 evident that something is still required to explain these 

 preferential routes of the water in dry consolidated 

 soil. There are none of them in pulverised soil, and 

 this points to the probable advantage in "dry farm- 

 ing" of stirring the soil to a sufficient depth to retain 

 the whole of the rainfall, in addition to the usual 

 practice of stirring the surface .soil after rainfall to 

 form a mulch to check its loss by evaporation. 



JOH\ AlTKEN. 



Ardenlea, Falkirk, December 5. 



I On an Apparently Distinctive Character of the Genus 

 j Bufo. 



On p. 38 of Mr. G. A. Boulenger's valuable work 

 on the "Tailless Batrachia of Europe" it is stated of 

 the vertebral column of the .\nura that " In those 

 forms in which the vertebrae are procqglous the eighth 



