There are continuous or intermittent forces in nature which it is the task of physics and chemistry to discover and, if possible, to subject to the needs of human industry, for this must be the aim of all scientific research if it is to be fruitful. This is how air and water have been used as engines. Attempts have even been made to use the flow of the tides, and before building his flax-spinning machine, Philippe de Girard had made attempts in this direction which did not succeed, perhaps because he lacked perseverance or because his attention was drawn to research of a more certain and immediate result; the idea has been taken up again since then, still without success, or with insufficient success; but there is nothing to prove that we should not succeed one day.
Attempts have also been made, at various times, to use solar heat, either as a caloric or as a motive force. It does not require a great clerk to guess that this has been achieved theoretically; but as the experiments were carried out in those regions where science and industry are in a constant state of progress, but which, on the other hand, are subject to the most bizarre caprices of temperature, it was necessary to give up the idea of taking advantage of a force which thick clouds seem to take a malicious pleasure in paralysing at any moment.
However, there are scientists who are not put off by anything, inventors who are not discouraged. M. Mouchot, currently a teacher at the Lycée de Tours, is one of them. For twenty-five years now, he has not been groping, but demonstrating the usefulness of the industrial application of solar heat. He has received encouragement, albeit somewhat platonic, from the Academy of Sciences, and the public knows that he is dealing with a serious man and a remarkable invention. But the invention remains there; the ingenious apparatus imagined by Mr. Mouchot enjoys the same success of esteem as a quantity of laboratory apparatus, undoubtedly giving very curious results, but without practical application; the anecdotal press likes to represent the inventor waiting for the occasion of a good ray of sunshine, if necessary for several days, to cook the chop for his lunch. Well, that is not enough; the invention and the inventor deserve much better than that.
If, once again, our regions are not favoured by the sun as much as it would be desirable, there are others with which it does not make so much of a fuss: and it is precisely in these that fuel is lacking, that drinking water, which would become distilled, is rare and that the use of machines is almost unknown.
Extreme heat," says M. Mouchot very well, "is as much a cause of desolation as the most intense cold. Under a sky of fire, man and animals lose their energy; water is most often lacking, either because it is only found at great depths, or because it forms, as in the Sahara, underground rivers flowing in the sand at a short distance from the ground. At the same time, the vegetation disappears or is only visible in places: its debris no longer provides the fuel necessary for life, and it is thus that vast regions remain closed to man or at most serve as a refuge for half-wild tribes. It is therefore especially at this time, it cannot be denied, that it is advisable to use the sun's rays, to dam up in some way this devastating force, and to make of it for the human race, instead of a scourge, one of its most powerful auxiliaries. And Mr. Mouchot proposes to do just that.
There is still something else. In the high regions of the atmosphere, which have been explored in recent times with unprecedented courage and audacity, it is known that the rarefaction of air is opposed to combustion. But could this source of heat, which is always there, not be used? Are our aeronaut physicists necessarily confined to meteorological studies? To ask such questions, as we say in the Chambers since there are Chambers, is to solve them. A Mouchot apparatus would not fit into an aerostat basket, and we still do not know what we could get out of it.
In the construction of his apparatus, M. Mouchot was guided by the well-known principle, which is also the basis of the construction of the greenhouses in our gardens: light and luminous heat pass through the glass, but obscure heat does not penetrate it; so that the luminous heat of the sun's rays which have penetrated the greenhouse through the glass covering it condenses there, because, having become obscure, it is impossible for it to pass through the glass again to escape. The melon bell is an even simpler demonstration of the phenomenon, but it is useless to insist on it. Mr. Mouchot's device is not much more complicated than the melon bell. It is a sort of lampshade lined internally with shiny metal sheets, but inverted, since the aim is the opposite of that of the ordinary lampshade placed over the light of a lamp. The sun's rays falling straight down on this funnel-reflector are concentrated at the central focus, occupied by a glass tube quite similar to the small chimney of the lamp, if the latter had been cut at the level of the narrow part, lower here, of the shade, and lined with a black interior envelope, to retain the stored heat even more maturely.
This heat thus stored becomes such that one can easily, with its help, prepare one's coffee, cook meat or any other kind of food, distil alcohol or, with the help of a boiler improvised with a saucepan or a cup, put a small machine in motion. These kinds of experiments have been taking place for several years in the courtyard of the Lycée de Tours, and can be seen, sunshine permitting, in the park of the Champ-de-Mars; drawings of the apparatus are on display in the liberal arts gallery. Another apparatus works, always in favourable weather, at the Trocadero. Water placed in a closed vessel in the reservoir of the apparatus easily reaches 153 degrees centigrade, and forty minutes are enough to bring a litre of water to boiling point. Finally, the apparatus can evaporate 5 litres of water per hour, and as long as the sun is not hidden by a curtain of clouds or has not disappeared from the horizon, it will be possible to operate a machine with the steam thus produced.
In 1877, M. Mouchot was part of a scientific mission sent to Algeria. He took advantage of this to carry out experiments which the climate of this country facilitated, and which were indeed decisive. His apparatus enabled him to bake bread, cook eggs, potatoes and meat, distil the juice of figs, from which, as in Greece, a kind of brandy is made, and finally to vaporise water in sufficient quantity for the steam to be used as a motive force. These results were communicated to the Academy of Sciences in May or April 1878, and the General Council of Algiers voted, as an encouragement to the inventor, a sum of 5,000 fr. We hope that the Exhibition of 1878 will bring him more, that is to say, the only reward the inventor aspires to, which is the serious application of his invention.
The Universal Exhibition of 1878
A memory has just brought us back to the Trocadero; we will take advantage of this to tell the reader about the most recent invention. More practical than the telephone, whose regular operation will always be at the mercy of atmospheric disturbance, and far superior to the phonograph, which is only a curiosity, this invention has for its object the practical use of solar heat.
In other words, Phoebus-Apollo reduced to the state of fuel and competing with the coal of Paris.
Ah! if, from heaven, his final resting place, he looks down on our poor planet, how dissatisfied with himself he must be who wrote:
Nihil humani a me alienum puto.
This is no longer what we should say today; today we have the right to exclaim:
Nihil divini a me alienum posco.
Indeed, where will man's progress stop? Franklin took the lightning; the aeronauts took the sky, and now Mr. Mouchot takes the sun.
A specialist who signs excellent articles under the modest pseudonym of "Un Ingénieur" has described M. Mouchot's apparatus in the newspaper La France, according to the conference given on this subject by M. Pifre at the Trocadero Palace:
"M. Mouchot had a large flared funnel built, made of brass, polished on the inside. It is easy to understand that a beam of solar rays falling on this conical mirror will be reflected and will come to cut itself on the axis of the funnel.
"On this axis, M. Mouchot places a small cylindrical vase of thin copper, blackened on the outside, which contains the material to be heated.
"The reflected solar rays are absorbed by the blackened surface of the copper vase, and the material placed inside can heat up. But as it heats up, it will in turn give off heat, and consequently cool down very quickly, if we do not take the precaution of surrounding the copper vessel with a glass cylinder.
A fact then occurs which requires some details and which we shall explain briefly by characterising it in one word: it is the theory of the melon bell.
There are two kinds of heat: the heat of the sun, of a fire, of a flame and that of a vase containing boiling water, that of a bar of iron heated to white and that which this bar of iron emits when it is still hot and is no longer red. To put it in the right terms, there is luminous heat and obscure heat.
Now, glass, rock crystal and ice allow the dark heat rays to pass through perfectly. If, therefore, we place plants in a garden under glass cloches, or in a greenhouse, or in a glass frame, these plants will receive a certain amount of luminous heat from the sun, which will warm them up. Being hot in their turn, they will emit heat which will be obscure and which, consequently, will not pass through the glass which surrounds them. Hence this rise in temperature, which is very easy to observe with the hand under glass frames, whatever they may be.
In this respect, water has properties similar to those of glass. This explains the rise in temperature experienced by the mud at the bottom of shallow ponds. This may also explain, to a certain extent, why sea water can be maintained at a relatively high temperature, despite the enormous amount of heat expended by evaporation at the surface.
"Finally, the water vapour in our atmosphere has the same effect on the heat absorbed by the earth. The humidity of the atmosphere thus has the effect of slowing down the cooling of our globe.
"The effect of the glass sleeve which surrounds the copper vase in M. Mouchot's apparatus is immediately understood, and it is easy to see that by accumulating heat at one point and preventing it from being lost outside, he was able to obtain from it what he desired.
"It is not necessary to insist much on the importance of this discovery in order to understand all that can be expected from it.
"The first result obtained was the fusion of lead and tin; then, later, the boiling of water. It is not surprising that water was only tried in the second place, for it takes almost three times as much heat to reduce a gram of ice to steam as to melt a gram of lead.
M. Mouchot reasoned as follows:
"Extreme heat is as much a cause of desolation as the most intense cold. Under a sky of fire, man and animals lose their energy; water is most often lacking, whether it forms, as in the Sahara, underground rivers flowing in the sand at a short distance from the ground. At the same time, the vegetation disappears and only appears in places: its remains no longer provide the fuel necessary for life, and this is how vast regions remain closed to man or at most serve as a refuge only for half-wild tribes. It is therefore especially then, it cannot be denied, that it is advisable to use the sun's rays, to dam up in some way this devastating force, and to make of it for the human race, instead of a scourge, one of its most powerful auxiliaries.
The experiments carried out proved M. Mouchot right. In 1877, on a scientific mission to Algeria, he used his apparatus to make bread, cook eggs, potatoes and meat, and to distil the fig juice used to make eau-de-vie.
We will end with a detail that will give the reader an even higher idea of the inventor. It is nothing less than using the heat of the sun as a driving force.
Indeed, water placed in the tank of the device easily reaches 153 degrees centigrade, and forty minutes are enough to bring a litre of water to the boil.
The device can evaporate 5 litres of water per hour.
Therefore, as long as the sun is not hidden by a curtain of clouds or has not disappeared from the horizon, it will be possible to operate a machine with the steam thus produced.
Mr. Victor Meunier, on the other hand, gave in the Rappel information that is a little more technical and that will complete those we have just quoted:
"You know that physicists call a calorie the quantity of heat necessary to raise the temperature of a litre (or kilogram) of water by one degree (centigrade). Well, in Algiers, in April, the heat supplied by each square metre of solar mirror was 7 calories per minute; in May, it was 8 calories; in June and July, 8.5.
"Expressed in everyday language, this is equivalent to saying that 1 litre of water at 20 degrees (room temperature) being put into the container of a solar boiler whose mirror has 1 square metre of surface, this litre of water is brought to 27 degrees at the end of the first minute, to 34 at the end of the second, and finally boils in twelve minutes, free of charge. In one hour, the same reflector draws from this litre of water 1,322 litres of steam weighing 778 grams. This with a small mirror; and according to the author's experiments, the results are increased by half when large reflectors are used.
"The following are now the quantities of heat collected in various places in Algeria at the times that will be indicated, as terms of comparison:
Sétif..........1er août 8h..............8.7
Constantine . 3 August 10am. (veiled sky) . 7.4
Biskra........10 August 10 h..................9.O
Touggourt peak 23 August 10 h. (2,100m high) 9.0
Médéah........16 Sept 7h................7.7
Lagouat.......22 Sept 10am..................8."
Gerville...... 4 Oct 8am................8.7
Hauts-Plateaux. 6 Oct 10am..............9.4
Oran...........23 Oct 3h..............1.8
"I believe I have succeeded, in these excursions, in popularising small solar appliances for cooking food and bread, distilling spirits, etc."
I extract this from a note by M. Mouchot to the Academy of Sciences. Indeed, the Algerians would have to be singularly at odds with themselves for the success that the professor from Tours boasts of having achieved not to have been definitively achieved after the above figures.
"For these countries and even for many others of a higher latitude, and even more so for those of a lower latitude, and therefore for a very large part of the surface of the globe, the invention of M. Mouchot is therefore equivalent to the discovery of innumerable and immense deposits of coal, (what am I saying here!) to the discovery of a universal and inexhaustible supply of fuel. And this is only one side of the coin. It should be noted that this amount of heat is free, that is to say, it does not give rise to any exploitation expenses, that it is used without work and that it goes by itself where it is needed. Is that all? Add to this the fact that with this improbable heating process there is no smoke or smell, that the appliances do not clog up, etc... But in the face of the above-mentioned advantages, these, despite their importance, become quite negligible. If one day the tropics do not erect statues to Mr. Mouchot as a benefactor, the tropics will be ungrateful.
©Les Merveilles de l'Exposition de 1878