1834] 



INVESTIGATION OF THE SPECIFIC HEATS OF ELASTIC FLUIDS. 



133 



M. Desbassyns de Riehemond, in the Island of Ascension; the 

 other is from observations by myself; and is among the number 

 of facts which I registered during my residence of several years 

 at the mines of Marinate 



In the Island of Ascension there vas an excellent spring, 



situated at the foot of a mountain originally covered with wood ; 



this spring became scanty and dried up, after the trees which 



covered the mountain had been felled. The loss of the spring 



.was rightly ascribed to the cutting down of the timber. The 



mountain was, therefore, planted anew, and a few years afterwards 



the spring reippeared by degrees, and by and by flowed with its 



former abundance. 



***** * * * 



We have still to inquire, whether extensive clearings of the 

 forest — clearings which embrace a whole country — cause any 

 diminution in the quantity of rain that falls. Unfortunately, the 

 observations which we have upon the quantity of rain which falls 

 in particular districts, are only of sufficient antiquity and accuracy 

 in Europe, to be worthy of any confidence, and there the soil was 

 cleared, before observation, in the generality of instances, began. 



The United States of America, where the forests are disap- 

 pearing with such rapidity, will probably one day afford elements 

 for the complete and satisfactory solution of the question, whether 

 or not the cutting down of forests causes any diminution in the 

 quantity of rain which falls in the course of the year. 



In studying the phenomena accompanying the fall of rain i B 

 the tropics, I have come to a conclusion which I have already 

 made known to many observers. My own opinion is, that the 

 felling of forests over a large extent of country has always the 

 effect of lessening the mean annual quantity of rain. 



It has long been said, that in equinoctial countries the rainy 

 season returns each year with astonishing regularity. There can 

 be no doubt of the general accuracy of this observation, but the 

 meteorological fact must not be announced as universal and 

 admitting of no exception ; the regular alternation of the dry and 

 rainy season is as perfect as possible in countries which present 

 an extreme variety of territory. Thus, in a country whose surface 

 is covered with forests, and rivers, and lakes, with mountains, and 

 plains, and table lands, the periodical seasons are quite distinct. 

 But it is by. no means so where the surface is more uniform in 

 its character. The return of the rainy season will be much less 

 regular if the soil be in general dry and naked; or if extensive 

 agricultural operations take the place of the primeval forest; if 

 rivers are less common, and lakes less frequent. The rains will 

 then be less abundant ; and such countries will be exposed, from 

 time to time, to droughts of long continuance. If on the con- 

 trary, thick forests cover almost the whole of the territory, if its 

 rivulets and rivers be numerous, and agriculture be limited in 

 extent, irregularity in the seasons will then take place, but in a 

 different way ; the rains will prevail, and in some seasons they 

 will become as it were incessant. 



The facts which have now been laid before the reader seem to 

 authorize me to infer — 



1st. — That extensive destruction of forests lessens the quantity 

 of running water in a country. 



2nd — That it is impossible to say precisely whether this dimi- 

 nution is due to a less mean annual quantity of rain, or to more 

 active evaporation, or to these two effects combined. 



3d. — That the quantity of running water does not appear to 

 have suffered any diminution or change in countries which have 

 known nothing of agricultural improvement. 



4th. — That independently of preserripg running streams, by 



opposing an obstacle to evaporation, forests economizo and reg»- 

 late their flow. 



5th. — That agriculture established in a dry country, not 

 covered with forests, dissipates an additional portion of its running 

 water. 



6th. — That clearings of forest land of limited extent may causo 

 the disappearance of particular springs, without our being there- 

 fore authorized to conclude that the mean annual quantity of rain 

 has I een diminished. 



7th, and lastly. — That in assuming the meteorological data 

 collected in intertropical countries, it may be presumed that 

 clearing off the forests does actually diminish the mean annual 

 quantity of rain which falls. 



Investigation of tiu Specific Heats of Elastic Fluids. 



BY M. V. REGNAUXT. 



I have been employed for more than twelve years in collecting 

 the elements necessary for the solution of the following general 

 problem : — ■ 



"A certain quantity of heat being given, what is theoretically 

 the moving force which can be obtained from it by applying it 

 to the developement and dilatation of different elastic fluids, iu 

 the various circumstances which can be realised in practice ?" 



The complete solution of this problem would give the true 

 theory, not only of the steam engines now in use, but also that of 

 engines in which the vapour of water was replaced by other 

 vapours, or even by a permanent elastic fluid, whose elastic force, 

 should be augmented by the heat. 



At the time I began these researches, the question appeared to 

 be more simple than it does at present. Starting from ideas then 

 admitted in science, it was easy to define clearly the different 

 elements which compose it, and I imagined processes by aid of 

 which I hoped to succeedi in finding in succession their laws, and 

 fixing their numerical data. But, as usually happens in the 

 sciences of observation, as I proceeded in my studies, the circle 

 continually augmented ; the questions which, at first, seemed to 

 me most simple, became quite complicated, and, perhaps, I should 

 not have had the courage to attack the subject, if, at the beginning, 

 I had understood all the difficulties. 



It has been admitted, until lately, that the quantities of heat 

 disengaged or absorbed by the same elastic fluid were equal, when 

 the fluid passed from the same initial to an identical final state, in 

 whichever direction the transition took place ; in a word, it was 

 admitted that the quantities of heat depended only on the initial 

 and final conditions of temperature and pressure, and were inde- 

 pendent of the intermediate circumstances through which the fluid 

 passed. S. Carnot published, in 1834, under the title of Reflec- 

 tions on the Motive Power of Fire, a work, which did not at 

 first excite much attention, and in which he admits, as a principle, 

 that the motive force produced in a fire engine is due to the 

 passage of the heat from the more heated calorific source which 

 emitsthe heat, to the cooler condenser which finally collects it. 



Mr. Chapeyronhas developed mathematically the hypothesis of 

 Carnot; and he has shown that, the quantities of heat gained or 

 lost by the same gas, then, do not depend solely upon its initial 

 and final state, but also upon the intermediate state through which 

 it is made to pass. 



The mechanical theory of heat has regained favour within a 

 few years, and it now employs a great number of mathematicians , 

 But the principle of Carnot has undergone an important modifi- 

 cation — it ha? been admitted that heat may hs transformed into 



