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1. iIoii(ureT\m]eS; the most ancient philosopher of Europe, propounded in Greece the 

 doctrine thut water is the origin of all organic beings a truth which even modern philosophy 

 cannot sliake in its principles. It is, tlerefore, excusable, if I put moisture at the head of 

 factors, althougli temperature, light and all their consequences are almost inseparable from it. 



Moisture affects vegetation in various shapes, viz., visible or invisible vapours in the atmos- 

 phere, such as fogs, dew, etc., or in the shape of water, such a& rain or snow. For practical 

 purposes the observations might be reduced to : 



(1.) The reading of the hygrometer at three periods of the day, Tiz., just before sunrise, 

 at noon (or rather at 1 P. m.) and after sunset. 



(2.) The inspection of the gauge, in order to ascertain the amount of rainfall (only once 

 a day). The information derived will necessarily be only approximate, for gauges at small 

 distances from one another will often give very different results. To equalize such uncertainty 

 the number of rainy days and observations of tlie clouds must be recorded. 



(3.) The notice of the frequency of rain, fog and dew. 

 These three points will suffice for the liygrometrical requirements of tropical forestry. 



2. Temperature. The thermometrical readings will have to be taken synclirouously 

 with the hygi-ometrieal readings, but it is highly necessary also to record the maximum and 

 minimum of the day. Besides this, observations of clouds and haze will be useful, as they mo- 

 derate the temperature considerably. Clouds intercept the solar rays during tlie day and 

 produce coolness, while during the night they intercept the radiation of insolated heat, and 

 retard the cooling of the atmosphere. Cloudy climates, therefore, have comparatively cool 

 days but warm nights. I think that three ciphers are quite sufficient for our purposes to 

 indicate the state of the sky, viz. 



0. Clear, or only with few clouds. 



1. Cloudy, more or less, from ^ to | of the sky. 



2. Cloudy all over, or at least more than half. 



If clouds alternate with clear sky, which takes place chiefly during the rainy season, it 

 might be expressed by 0' x 1 or 0' x 2, as the case may be. 



The change from the cold to the hot season, in excessive tropical climates, is usually very 

 sudden, so much so, tliat we can readily fix the commencement of the hot weather 

 within a few days. This rapid change is chiefly due to the shedding of the leaves of the 

 trees, by which act the evaporation as well as the precipitation of moisture is so quickly dimin- 

 ished as to become sensible even to a superficial observer. In an evergreen forest, or in 

 regions well stocked with evergreens, such is not the case, and here the hygrometrioal 

 changes are more gradual and regular. Tlie commencement of the hot season greatly depends 

 upon the amount of rainfall of the foregoing rainy season, and sets in earlier if the 

 rainfall during that period was moderate ; but on impermeable exposed strata trees always 

 shed their leaves earlier. Tlie immediate cause of leafshedding is, in temperate climates, 

 attributed to the action of cold,* but in the tropics, it appears to me to be intimately connected 

 with the supply of moisture, for otherwise we could not explain why the same tree on a ridge 

 should shed its leaves, while its neighbour a few fathoms below, in a moister situation, should 

 retain its foliage for weeks and even mouths longer. 



The intensity of solar rays is a subject of interest to a tropical forester, because it affects 

 the growth of trees to a certain extent. Observations should therefore be taken with a black- 

 bulb thermometer constructed on the principle of a maximum thermometer. Tlie intensity 

 of the solar rays in their illuminating as well as in their calorifying qualities, inversely increases 

 with the density of the atmosphere ; hence it happens that one may so easily catch a cold on 

 the summit of a lofty mountain, if one seeks protection from the rays of the sun under the 

 shade of trees. 



3. Other factors which affect climate are especially wind and erpomre. Notes on the gen- 

 eral direction of the winds, etc., are, therefore, useful, always supposing that we also know the 

 character of the tracts of land or waters over which these winds have travelled, so as to enable 

 US to bring into account their heating or cooling, and their drying or moistening effects. The 

 exposure of the station of observation should always be noted in the same way as its elevation. 

 If it is freely exposed on a plain or at the bottom of a valley, it will be sufficient to use the 

 terms. //re" or sheltered. 



All these observations will have to be entered in tabular forms from which, after a few 

 years' observation, a general diagnosis of the respective climates may be deduced. 



* Dr. Inman (Proceed, of Liverpool Phil. See. 1844-45 p. 133, sqq.) and Dr. Lcdeganck (Bulletin 

 of the Belgian Bot. Soc. X. p. 13H, sqq.) ascribe the act of leafshedding to cold which operates by causing 

 H greater contraction of the half-dead spongy tissues of the petiole than of the tense cushion, and so ruptures 

 the cells. I think it more probable that expansion takes place instead of conti-action , for water forms a 

 most remarkable exception to the regular laws of expansion by heat. Water acquires its greatest density at 

 about 39.5 Fahr. Botli heat above and cold below this point cause expansion. This anomalous expansion of water 

 is productive of most important cousequences iu nature (see Tomliuson's Introduction to the study of Natural 

 Philosophy). 



