TSIOLKOVSKIJ, K(ONSTANTIN E(DUARDOVICH). ) - 
TSIOLKOVSKI, TSIOLKOVSKIY, TSIOLKOWSKY, CIOLKOWSKY, ZIOLKOWSKY, TZIOLKOWSKY.

(Moskva, 10th of July, 1904). 4to. (26 x 21,5 cm). Unbound, in cloth box. 4 pp. (= pp. 1-4). Well preserved.

Rare original article, printed before it was published in the newspapers and in periodicals, by fourteen engineers and scientists defending the inventions by Tsiolkovski. This article gives a very interesting and rare insight into the circumstances, under which T. was forced to publish his works and the rude lack of recognition he received. The article must be virtually impossible to get a hold of.

Having examined all known attempts to construct a safe dirigible, and realized taht none of them could work, T. came up with his own "breathing" all-metal dirigible; he developed his idea fully and did not present it, till everything was perfect. The idea was received with curiosity, but the conclusion (by the Russian Technical Society) was that "the aerostat (...) is doomed for ages, by the very nature of things, to remain a plaything for the wind." (Collected Works, 1959, III: p. 6). None the less, T. continued publishing his theories in a number of pamphlets and articles from 1892 - 1908. The response was varying, but in 1904 his proposals were discussed in Kaluga by a group consisting of 14 engineers. "They concluded, among other things, that his project was definately feasible, highly important and indisputably correct from the theoretical standpoint. Following is the text of a press report of these conclusions: "The author of this project, as his numerous published works demonstrate, has rigorously and comprehensively studied and elaborated the entire theory of aeronautics, carried out a great deal of mathematical experimental research in this field, weighed all the known principles of aeronautics, and in drafting his project, guided himself only by incontestable principles established on the airship of K.E. Tsiolkovskiy is the result of persistent toil and the zealous pursuit of an idea. it is to be hoped that these remarks will not pass unnutoced and that both the public and the press will remember the existence of this project of our compatriot Tsiolkovskiy and support its materialization. (Note: The conclusions of the group og engineers in Kaluga were published separately on page 4 (Kaluga, 10 July 1904) and immediately afterward printed in various Russian newspapers and periodicals). This is followed by the signatures of 14 mechanical engineers, technologists, railroad engineers, and candidates of mathematical sciences." (Collected Works, 1959, III: p. 7-8). The article was still ineffective regarding official recognition of his works, and at most provided the author with moral support.

After the Revolution T. became a member of the Academy, was allotted a personal pension in 1921, and became able to devote himself entirely to his scientific work. Now his contemporaries finally saw him as the founder of a new domain of human knowledge, a new science, he was, and he was awarded the Red Banner of Labour Order for outstanding services to his country.

1. Dopolnitjenyja tjekhizsjeskija dannija k postrojeniju metallizsjeskoj obolozski dirischablja bjez dorogoj verfi. 2. Otzyv Ljeljenpovskago Obschtstva o mojem dirischabl. Izdanije litsa, poschjelavschtsago ostatcja njeizvctnym (Russian). (1. Additional Technical Data on the Construction of Metal Envelope of Dirigible Without Expensive Dock. 2. The Attitude of the Ledentsovskiy Society towards My Dirigible. Published by a Person, who Wishes to Remain Unknown).

Kaluga, 1915. 8vo. (25,5 x 17 cm). Orig. printed, illustrated green wrappers, inside af wrappers also illustrated w. figures. Faint fading at edges, otherwise mint condition. Illustr. 10 pp. (= pp. 1-10).

Exceedingly rare and important first edition (in Russian).
As T. in 1919 became a member of the Socialist Academy (later cooperated in the USSR Academy of Science), recognition and money enabled him to publish a much larger quantity of articles and books than he had done before. Everything by T. published before 1918 was paid for by himself; his modest salary only allowed him to pay for few copies of each, and four times as many of his articles, booklets and books were published after 1918 as in all the years before; each one after 1918 was likewise issued in a much greater number. "During a period of 26 years before the revolution his publications numbered less than 50." (Collected Works, NASA, 1951, I; p. X). This together with the fact that his work was not recognized and his experiments discredited, means that probably very few copies were preserved. His early works are thus of the greatest rarity. "Like all the pre-1918 papers by Tsiolkovsky, which were printed on his own expense in very small numbers only, that virtually have disappeared and the re-occurrence of one of them is very remarkable." (Catalogue 282, Interlibrum Vaduz).
Elaborately presenting the metal-zeppelin as the first, T. presented it in detail in this work. Had construters of the zeppelins only studied this work by T., many disasters could have been avoided.

TSIOLKOVSKIJ, K(ONSTANTIN E(DUARDOVICH)
TSIOLKOVSKI, TSIOLKOVSKIY, TSIOLKOWSKY, CIOLKOWSKY, ZIOLKOWSKY, TZIOLKOWSKY.

Davljenije na ploskost pri jeje normalnom dvischjekii v vozdukhje (Russian). (The Preassure on a Plane in Normal Motion in Air).

Kaluga, 1930. (12mo 14,5 x 10 cm). Orig. printed blue wrappers. Very well preserved. 49 pp. (= pp. 1-49).

First edition. All works by Tsiolkovsky are rare and important.
Tsiolkovski was a pioneer in the field of experimental aerodynamics; by, among other things, building one of the world's first wind-tunnels, he derived a number of highly important laws of aerodynamics.T. is one of the founders of modern aerodynamics.

TSIOLKOVSKIJ, K(ONSTANTIN E(DUARDOVICH)
TSIOLKOVSKI, TSIOLKOVSKIY, TSIOLKOWSKY, CIOLKOWSKY, ZIOLKOWSKY, TZIOLKOWSKY.

Kaluga, 1928 + 1930. Both 8vo. (26 x 17,5 cm. + 25 x 17,5 cm). Both in the orig. printed wrappers. Minor soiling to front-wrappers, but both works are well preserved. The first wok contains 4 pp. and the Atlas 15 pp. almost entirely with illustrations. 17 pp. (= pp. 1-17) + 24 pp (= pp. 1-24).

Two very rare and important, ricly illustrated first editions (in Russian).
One of the works of T. that give the absolute best idea of, how he imagined his groundbreaking metal-dirigible. Few of his works are as richly illustrated as these two. "The works of Tsiolkovskiy that date from the Soviet period repeat the principle assumption of his theory of the metal aerostat, provide counsel on the practical organization of experimental work, survey critically the existing types of dirigibles, and point out the errors in the research into metal-envelope dirigibles in the USSR and abroad." (Collected Works, 1959, III: p. 23).

 TSIOLKOVSKIJ, K(ONSTANTIN E(DUARDOVICH).
TSIOLKOVSKI, TSIOLKOVSKIY, TSIOLKOWSKY, CIOLKOWSKY, ZIOLKOWSKY, TZIOLKOWSKY.

Kaluga, 1931. Small 8vo. (15,5 x 11 cm). Orig. purple printed wrappers. A few small holes to the last leaves, not affecting text, otherwise very well preserved. 32 pp. (= pp. 1-32).

Rare first edition (in Russian).
"The works of Tsiolkovskiy that date from the Soviet period repeat the principle assumption of his theory of the metal aerostat, provide counsel on the practical organization of experimental work, survey critically the existing types of dirigibles, and point out the errors in the research into metal-envelope dirigibles in the USSR and abroad." (Collected Works, 1959, III: p. 23).

TSIOLKOVSKIJ, K(ONSTANTIN E(DUARDOVICH)
TSIOLKOVSKI, TSIOLKOVSKIY, TSIOLKOWSKY, CIOLKOWSKY, ZIOLKOWSKY, TZIOLKOWSKY.

Gondola metallizsjeskago dirischablija i organy ego uprabljenija (Russian). (The Gondols of the Matal Dirigible and its Controls."

Kaluga, (1918). 8vo (26,8 x 18 cm). Orig. printed illustrated wrappers, inside of wrappers also illustrated w. figures. One text-illustration. Uncut and unopened. A few tears to extremities and some fading. 24 pp. (= pp. 1-24).

Exceedingly rare and interesting first edition (in Russian). Early and elaborate description of Tsiolkovski's famous, huge transport dirigible. He is the inventor of the all-metal dirigible, which, in contrast to all other dirigibles at the time, would not be inflammable and which would be albe to keep the gas inside it; he found it possible to heat the hydrogen artificially, thus providing the dirigible with constant and perfect vertical maneuverability. No other dirigibles than his could be controlled by man.

As T. in 1919 became a member of the Socialist Academy (later cooperated in the USSR Academy of Science), recognition and money enabled him to publish a much larger quantity of articles and books than he had done before. Everything by T. published before 1918 was paid for by himself; his modest salary only allowed him to pay for few copies of each, and four times as many of his articles, booklets and books were published after 1918 as in all the years before; each one after 1918 was likewise issued in a much greater number. "During a period of 26 years before the revolution his publications numbered less than 50." (Collected Works, NASA, 1951, I; p. X). This together with the fact that his work was not recognized and his experiments discredited, means that probably very few copies were preserved. His early works are thus of the greatest rarity. "Like all the pre-1918 papers by Tsiolkovsky, which were printed on his own expense in very small numbers only, that virtually have disappeared and the re-occurrence of one of them …is very remarkable." (Catalogue 282, Interlibrum Vaduz). 

TSIOLKOVSKIJ, K(ONSTANTIN E(DUARDOVICH)
TSIOLKOVSKI, TSIOLKOVSKIY, TSIOLKOWSKY, CIOLKOWSKY, ZIOLKOWSKY, TZIOLKOWSKY.

Kaluga, 1914. 8vo. (25,8 x 17,5 cm). Original printed, illustrated wrappers. Some tears to wrappers, but most of it intact, only lacking parts of corners. Internally some tears to edges and corners, but none affecting text or illustration. Illustrated w. one figure ("Skhema "pakjety" - a cutthrough of the rocket).

Exceedingly rare and important first edition (in Russian). As T. in 1919 became a member of the Socialist Academy (later cooperated in the USSR Academy of Science), recognition and money enabled him to publish a much larger quantity of articles and books than he had done before. Everything by T. published before 1918 was paid for by himself; his modest salary only allowed him to pay for few copies of each, and four times as many of his articles, booklets and books were published after 1918 as in all the years before; each one after 1918 was likewise issued in a much greater number. "During a period of 26 years before the revolution his publications numbered less than 50." (Collected Works, NASA, 1951, I; p. X). This together with the fact that his work was not recognized and his experiments discredited, means that probably very few copies were preserved. His early works are thus of the greatest rarity. "Like all the pre-1918 papers by Tsiolkovsky, which were printed on his own expense in very small numbers only, that virtually have disappeared and the re-occurrence of one of them is very remarkable." (Catalogue 282, Interlibrum Vaduz).

This is one of Tsiolkovski's most important works. It is written at a time when the lack of recognition together with the realization that his discoveries were of groundbreaking character became almost too much for him. He begins the article by writing "It is difficult to work alone and in adverse circumstances for many years without receiving recognition or encouragement from anyone. From the articles concerning "rockets" it is clear that with our present technology we are still far from attaining the required velocities. Now, however, I would like to popularize my ideas, to offer explanations, and refute the view that the "rocket" is something far beyond our grasp. Below are some of the theorems I have already proved; here I shall dwell on those that may seem not entirely convincing." The front-wrapper shows a section through his rocket, also with an astronaut.

"From 1903 to 1917 Tsiolkovsky offered several plans for constructing rocket ships. He considered such questions as guiding a rocket in vacuum, the use of a fuel component to cool the combustion chamber walls, and the application of refractory elements." (D.S.B. XIII:483).
His theories on rocket propulsion are some of his most vital and progressive of his discoveries. For his methods of studying the dynamics of rocket flight and his approach to interplanetary travels he has, after much struggle, won world recognition and is now widely known. "Tsiolkovskiy pushed back the frontiers of human knowledge, and his idea of using the rocket for the exploration of space is only now, in our own time (i.e. 1954), beginning to be fully appreciated. He was the father of the theory of long-range liquid-fuelled rockets and the founder of a vigorously scientific theory of inter-planetary travel." (Collected Works, NASA, 1954. II: p. 3). He was thus the founder of modern rocket dynamics. His works on rocket dynamics and the theory of interplanetary flight "were the first in world scientific literature. They preserve their scientific and practical significance to this day." (Collected Works, 1954, II: p. 7). He was the first in the history of science to calculate the efficiency of the rocket and to point out the advantages of rocket motors at high velocities.

"Today, his original research, in the course of which he not only predicted the development of jet aircraft and low-range rockets but theoretically confirmed their feasibility, is necessarily of great interest to the entire aerospace community. He was the founder of the science astronautics (cosmonautics), which is concerned with the problems of interplanetary travel, and he derived the classical formulas of rocket flight." (Collected Works, NASA, 1951. I: p. IX).

TSIOLKOVSKIJ, K(ONSTANTIN E(DUARDOVICH)
TSIOLKOVSKI, TSIOLKOVSKIY, TSIOLKOWSKY, CIOLKOWSKY, ZIOLKOWSKY, TZIOLKOWSKY.

(Kaluga, 1927). 8vo. (17 x 13 cm). No wrappers (none issued). One full-page illustr. of a rocket-motor. First two and last two leaves w. repairs to margins, otherwise fine. 24 pp. (= pp. 1-21).

Very rare and important work, published by the author himself. With original handwritten dedication in Russian for R(...?) "Nikolajevskij/ K. Tsiolkovskij/ 17 9. 1939 g." The work was issued without title-page and wrappers.
During the 1920's he elaborated his theories of multistage-rockets and began working out his theory of the flight of jet airplanes, devoting a number of papers to this. This is a work of great significance to T. himself. In his "Published Works" he later says about this work that it was an attempt to indicate the prepatory tests in the desining of a space-vehicle. As an appendix at the end of the work (pp. 22-24) there is an "Objection to Eng. Lademann", who was an engineer; he published a review of T.'s "Study of Outer Spaces" in April 1927, where he, among other things, very much doubted the proposal to fit the reactive flying vehicle with rudders at the rear. T.'s novel technical ideas were so forward-looking and ahead of his time that scientists with speciality in the same field were unable to appreciate them.

TSIOLKOVSKIJ, K(ONSTANTIN E(DUARDOVICH)
TSIOLKOVSKI, TSIOLKOVSKIY, TSIOLKOWSKY, CIOLKOWSKY, ZIOLKOWSKY, TZIOLKOWSKY.

Kaluga, 1929. 8vo. (21,5 x 15 cm). Original printed wrappers. Front-wrapper lacking small part of upper right corner, a few tears to extremities, otherwise fine. 38 pp. (=pp. 1-38) + a portrait of Tsiolkovski.

Rare and important first edition (in Russian).
During the 1920's he elaborated his theories of multistage-rockets and began working out his theory of the flight of jet airplanes, devoting a number of papers to this. This work deals withT.'s proposals for attaining maximum height and range with composite rockets. He foresees the difficulties flight to the stars will cause for the scientists and engineers, as flight to the stars cannot be compared with flight in air. "If they understood the difficuties, many now working with enthusiasm would abandon it." (p.8). 

TSIOLKOVSKIJ, K(ONSTANTIN E(DUARDOVICH)
TSIOLKOVSKI, TSIOLKOVSKIY, TSIOLKOWSKY, CIOLKOWSKY, ZIOLKOWSKY, TZIOLKOWSKY.

Kaluga, 1929. 8vo (17 x 12,5 cm). Orig. printed red wrappers. Minor dampstaining to upper corner of wrappers, not affecting text. Otherwise very well preserved. 38 pp. (= pp. 1-38) + 1 leaf w. illustrations.

Rare first edition (in Russian). Printed privately by the author. One of his most important works on rocket dynamics and interplanetary flight.
During the 1920's he elaborated his theories of multistage-rockets and began working out his theory of the flight of jet airplanes, devoting a number of papers to this.

TSIOLKOVSKIJ, K(ONSTANTIN E(DUARDOVICH)
TSIOLKOVSKI, TSIOLKOVSKIY, TSIOLKOWSKY, CIOLKOWSKY, ZIOLKOWSKY, TZIOLKOWSKY.

Kaluga 1930. Orig. printed wrappers. 8vo. (16,5 x 12,8 cm). A few spots to margins of wrappers, otherwise mint. 91 pp. + 21 pp. with illustrations only.

Rare and interesting first edition (in Russian). Few of T.'s works are as extensive and elaborate as this one. T. was clearly very much ahead of his time in the design of the metal dirigible presented here. Science and technology only very lately grasped his conceps and by the time they did, their author was forgotten and T.'s name was, very wrongly, not mentioned. "The works of Tsiolkovskiy that date from the Soviet period repeat the principle assumption of his theory of the metal aerostat, provide counsel on the practical organization of experimental work, survey critically the existing types of dirigibles, and point out the errors in the research into metal-envelope dirigibles in the USSR and abroad." (Collected Works, 1959, III: p. 23).

TSIOLKOVSKIJ, K(ONSTANTIN E(DUARDOVICH)
TSIOLKOVSKI, TSIOLKOVSKIY, TSIOLKOWSKY, CIOLKOWSKY, ZIOLKOWSKY, TZIOLKOWSKY.

Kaluga, 1930. Small 8vo (14,5 x 10 cm). Orig. printed wrappers, Lower right corner missing throughout, not affecting text. Otherwise very well preserved. 24 pp. (= pp. 1-24).

Rare and important first edition (in Russian). Tsiolkovski spent the last years of his life developing the theory of jet aircraft.
During the 1920's he elaborated his theories of multistage-rockets and began working out his theory of the flight of jet airplanes, devoting a number of papers to this. In this work he describes the advantages as well as disadvantages of jet-propelled aircraft compared with propellar-driven aircraft in great detail. He describes in detail how the velocity of the jet aircraft will be much higher and consume less energy than the propellar-driven aircraft, if it goes high enough. He ends the article with writing "The era of propellar-driven aircraft should be followed by an era of jet aircraft, or stratospheric aircraft" -and this is written ten years before the world's first jet-plane was launched. 

TSIOLKOVSKIJ, K(ONSTANTIN E(DUARDOVICH)
TSIOLKOVSKI, TSIOLKOVSKIY, TSIOLKOWSKY, CIOLKOWSKY, ZIOLKOWSKY, TZIOLKOWSKY.

Kaluga, 1927. Small 8vo (17 x 12,5 cm.) Orig. wrappers, back professionally repaired. Very well preserved. Illustr. w. three figures. 72 pp. (= pp. 1-72).

First edition. All works by Tsiolkovski are rare and important.
Tsiolkovski was a pioneer in the field of experimental aerodynamics; by, among other things, building one of the world's first wind-tunnels, he derived a number of highly important laws of aerodynamics and is one of the founders of modern aerodynamics. In 1921 he came up with the idea of making a vehicle of transportation that was carried on a cushion of air. It is in this work he develops the idea (see D.S.B. XIII: 484).

TSIOLKOVSKIJ, K(ONSTANTIN E(DUARDOVICH)
TSIOLKOVSKI, TSIOLKOVSKIY, TSIOLKOWSKY, CIOLKOWSKY, ZIOLKOWSKY, TZIOLKOWSKY.

Kaluga, 1932. Small 8vo (14,3 x 10,5 cm). Orig. printed wrappers, illustrated w. four figures on inside of wrappers. Very well preserved. W. stamp (3 kop.), poststamp (from Kaluga) and address written in hand (probably Tsiolkovski's own) on back-wrapper. 32pp (=pp. 1-32).

Rare and important first edition (in Russian). During the 1920's he elaborated his theories of multistage-rockets and began working out his theory of the flight of jet airplanes, devoting a number of papers to this. It is in this work (and in "The Rocketplane") he developes the theory of the liquid-fuel jet engine in an aeroplane and for the first time in technical literature describes the turbo-prop aircraft, -and in great detail. "... This is an indispensible contribution of Russian science to the common treasury of human culture." (Collected Works, NASA, 1954, II: p.17).

TSIOLKOVSKIJ, K(ONSTANTIN E(DUARDOVICH)
TSIOLKOVSKI, TSIOLKOVSKIY, TSIOLKOWSKY, CIOLKOWSKY, ZIOLKOWSKY, TZIOLKOWSKY.

Kaluga, 1915. 8vo. (25,5 x 17 cm). Orig. printed wrappers, illustrated w. figures on the inside. Back repaired. Small nicks and scratches to extremities. Tear to front wrapper professionally restored. In good condition. 22pp (=pp. 1-22).

Exceedingly rare and important first edition (in Russian).
As T. in 1919 became a member of the Socialist Academy (later cooperated in the USSR Academy of Science), recognition and money enabled him to publish a much larger quantity of articles and books than he had done before. Everything by T. published before 1918 was paid for by himself; his modest salary only allowed him to pay for few copies of each, and four times as many of his articles, booklets and books were published after 1918 as in all the years before; each one after 1918 was likewise issued in a much greater number. "During a period of 26 years before the revolution his publications numbered less than 50." (Collected Works, NASA, 1951, I; p. X). This together with the fact that his work was not recognized and his experiments discredited, means that probably very few copies were preserved. His early works are thus of the greatest rarity. "Like all the pre-1918 papers by Tsiolkovsky, which were printed on his own expense in very small numbers only, that virtually have disappeared and the re-occurrence of one of them is very remarkable." (Catalogue 282, Interlibrum Vaduz).
As can be gathered from the preface of this work, many people without influence in Russia were beginning to recognize the scientific validity and value of T.'s conclusions as well as the indisputable advantages, his airships would pose to tha national economy, should they be built. But dozens of years passed before anyone with influence and knowledge (in Russia or abroad) began realizing the value of T.'s proposals.

TSIOLKOVSKIJ, K(ONSTANTIN E(DUARDOVICH)
TSIOLKOVSKI, TSIOLKOVSKIY, TSIOLKOWSKY, CIOLKOWSKY, ZIOLKOWSKY, TZIOLKOWSKY.

Soprotivljenije vozdukha i vozdykhoplavanije. Ottisk iz schurnala "Hayzsnoje Obozrenije" (Russian). (Air Resistance and Aeronautics [or Air Resistance and Flight in Air]. Offprint from the Magazine "Scientific Review").

Kaluga, 1902 (colophon stating 1902, wrapper stating 1903). 8vo (23,5 x 15 cm). Uncut in the orig. printed blue-green wrappers. Professionally resored at corners. Internally brownspotted. Well-preserved copy. 22 pp. (= pp. 1-22).

Exceedingly rare first edition (in Russian). It was printed as an article in "Scientific Review" and an offprint was made, no other issues appeared, thus these wrappers must be the original, even though they state 1903 and the colophon 1902. As it was printed at Tsiolkovski's own expenses and he did not have much money, it is likely that he had the wrappers for the offprint printed the following year. A work of great rarity; all his subsequent works on this topic (aerodynamics) belong to the Soviet period.

Tsiolkovski created one of the world's first wind-tunnels ("vaned blower"). "In 1897 he constructed a wind tunnel, the first in Russia to be used in aviation." (D.S.B. XIII: 483). He experimented with it every day for more than four years. From these experiments he "was able to determine with remarkable accuracy such a small quantity as, for example, the air friction on the surface of a model and to establish the fundamental laws by which it is governed." (Collected Works, 1951, I: pp. 8-9). Having the modest salary of a teacher, these experiments cost him most of his money, and he tried to raise money for his experiment through the Russian Academy of Sciences. The examiner of the project, M.A. Rykachev, found it very interesting and of great importance, and for the fist time Tsiolkovski was granted money to continue his experiments (470 rubles); he spent all the money on building an even larger wind-tunnel. The results of these seminal experiments were published for the first time in this exceedingly rare article from 1902. To his disappointment he was granted no more funds and as suspicion from the other scientists and the tsar-regime rose towards him, the fundamental results attained in this work remained unacknowledged by the official scientific circles.

As Tsiolkovski in 1919 became a member of the Socialist Academy (later cooperated in the USSR Academy of Science), recognition and money enabled him to publish a much larger quantity of articles and books than he had done before. Everything by T. published before 1918 was paid for by himself; his modest salary only allowed him to pay for few copies of each work, and four times as many of his articles, booklets and books were published after 1918 as in all the years before; each one after 1918 was likewise issued in a much greater number. "During a period of 26 years before the revolution his publications numbered less than 50." (Collected Works, NASA, 1951, I; p. X). This together with the fact that his work was not recognized and his experiments discredited means that probably very few copies were preserved. His early works are thus of the greatest rarity. "Like all the pre-1918 papers by Tsiolkovsky, which were printed on his own expense in very small numbers only, that virtually have disappeared and the re-occurrence of one of them …is very remarkable." (Catalogue 282, Interlibrum Vaduz).

Tsiolkovski was a pioneer in the field of experimental aerodynamics; by, among other things, building one of the world's first wind-tunnels, he derived a number of highly important laws of aerodynamics. There is no doubt that T. is one of the founders of modern aerodynamics. 

SOURCE and CREDIT:  LYNGE RARE BOOKS DENMARK