Geometry.Net - the online learning center
Home  - Scientists - Clapeyron Emile

e99.com Bookstore
  
Images 
Newsgroups
Page 1     1-20 of 88    1  | 2  | 3  | 4  | 5  | Next 20
A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z  

         Clapeyron Emile:     more detail
  1. Abhandlung über die bewegende Kraft der Wärme, von E. Clapeyron by Emile Clapeyron, 1926-01-01

1. Livres Essais Architecture
Translate this page ECRITS D'INGENIEURS PERRONET JEAN-RODOLPHE , LAME GABRIEL , clapeyron emile chezDU LINTEAU. EMPIRE ET DECOR L'ARCHITECTURE ET LA QUESTION DE LA TECHNIQUE
http://www.archicool.com/amazon/essais.shtml
Livre anglais www.archicool.com
Essais La cyber librairie d'Archicool partenaire d' Ecrits auteurs A LA RECHERCHE DE L'ARCHITECTURE. ESSAI D'EPISTEMOLOGIE DE LA DISCIPLINE ET DE LA RECHERCHE
Stephane Hanrot
UN AMOUR SANS LIMITE
FREYSSINET EUGENE
chez DU LINTEAU APRES LE CUBISME
OZENFANT AMEDEE , JEANNERET CHARLES-EDOUARD
chez ALTAMIRA - ECLECTIC Les Architectes : mutations d'une profession
Guy Tapie
chez L'Harmattan (Logiques Sociales) L' ARCHITECTURE EN QUESTIONS : 15 ENTRETIENS AVEC DES ARCHITECTES
EMERY MARC , BRAUSCH MARIANNE
chez MONITEUR ARCHITECTURE ET CONCEPTS NOMADES
GIRARD
chez MARDAGA PIERRE ARCHITECTURE ET NATURE : CONTRIBUTION A UNE ANTHROPOLOGIE DU PATRIMOINE
MARTIN JEAN-BAPTISTE , LAPLANTINE FRANCOIS
chez PRESSES UNIVERS L' ARCHITECTURE MODERNE EST MORTE
MO BLAKE
chez MONITEUR L' Art du lieu
Norberg Schulz/Chris
chez Le Moniteur (Architextes) BALISE URBAINE, NOMADES DANS LA VILLE BOISCUILLE (DE) CHILPERIC chez DE L'IMPRIMEUR CETTE ARCHITECTURE QUI NOUS PARLE MAYEROVITCH HARRY chez MULTIMEDIA / ROBERT DAVIES CETTE MAISON ME RENDRA FOLLE GORON JOELLE chez FLAMMARION - FICTION LA CHASSE A LA BAGNOLE : REFLEXIONS SUR LE REJET DE L'AUTOMOBILE EN MILIEU URBAIN VAGNON HENRI chez L'HARMATTAN LA CITE N'APPARTIENT A PERSONNE : ARCHITECTURES, ESTHETIQUE DE LA FORME...

2. Kernion's Chemicals - Chapter Ten Review
Emile clapeyron emile Clapeyron was educated at the Ecole Polytechniquefrom which he graduated in 1818. He then went to the Ecole
http://www.mtlebanon.k12.pa.us/senior/science/mkernion/projects/chemistrysite/ap

3. References For Clapeyron
References for emile clapeyron. Biography in Dictionary of ScientificBiography (New York 19701990). Books E Mendoza (ed.), Reflections
http://www-gap.dcs.st-and.ac.uk/~history/References/Clapeyron.html
References for Emile Clapeyron
  • Biography in Dictionary of Scientific Biography (New York 1970-1990). Books:
  • E Mendoza (ed.), Reflections on the motive power of fire by Sadi Carnot, and other papers on the second law of thermodynamics by E Clapeyron and R Claissius (New York, 1960). Articles:
  • Ann. of Sci.
  • Math. Comput. Simulation
  • M Kerker, Sadi Carnot and the steam engine engineers, Isis
  • F Sebastiani, The caloric theories of Laplace, Poisson, Sadi Carnot and Clapeyron and the theory of thermal phenomena in gases formulated by Clausius in 1850 (Italian), Physis - Riv. Internaz. Storia Sci. Main index Birthplace Maps Biographies Index
    History Topics
    ... Anniversaries for the year
    JOC/EFR October 1998 School of Mathematics and Statistics
    University of St Andrews, Scotland
    The URL of this page is:
    http://www-history.mcs.st-andrews.ac.uk/history/References/Clapeyron.html
  • 4. Short Biographies: Ch
    clapeyron, Benoit Paul emile 26 Jan 1799 28 Jan 1864 French Educated at École Polytechnique he went to Russia in 1810, remaining there for 21 years. He expressed Sadi Carnot's ideas on heat analytically, with the help of graphical
    http://physics.hallym.ac.kr/reference/physicist/Ch.html
    Clapeyron , Benoit Paul Emile
    26 Jan 1799 - 28 Jan 1864
    French
    Sadi Carnot
    's ideas on heat analytically, with the help of graphical representations, in 1834. This work had important influences on Thomson and Clausius . He also was interested in railway engineering and designed steam locomotives. The Clapeyron relation, a differential equation which determines the heat of vaporisation of a liquid, is named after him.
    Clausius , Rudolf Julius Emmanuel
    2 Jan 1822 - 24 Aug 1888
    German
    1. The energy of the universe is constant.
    2. The entropy of the universe tends to a maximum.
    In all he wrote eight important papers on the topic.
    He restated Sadi Carnot 's principle of the efficiency of heat engines. The Clausius- Clapeyron equation expresses the relation between the pressure and temperature at which two phases of a substance are in equilibrium. JOC/EFR February 1995

    5. Die Clausius-Clapeyron-GleichungFür Das GGW Zweier Phasen Gilt , Und Außerdem
    Rearranging the letters of 'Benoit Paul emile clapeyron' (Mathematician) gives
    http://saftsack.fs.uni-bayreuth.de/thermo/claclap.html
    [Weiter] [Inhaltsverzeichnis] [Suchen] D IE C LAUSIUS- C LAPEYRON- G LEICHUNG Wegen das chemische Potential gleich der spezifischen freien Enthalpie . Entlang der Grenzkurve gilt also und somit ableiten. Dazu betrachtet man p als Funktion von T und differenziert nach T: Daraus ergibt sich die Clausius Clapeyron -Gleichung Dabei ist Beispiel: Dampfdruckkurve
    • Annahme 1:
    • Annahme 2:
    • Annahme 3:
    Damit ergibt sich: Daher gilt Man sieht daran: [Weiter] Systeme mit mehreren Komponenten
    [Inhaltsverzeichnis]
    [Suchen] [Kommentar] Online-Skript Thermodynamik und Statistische Physik

    6. Clapeyron
    Biography of Benoit Paul emile clapeyron (17991864) Benoit Paul emile clapeyron. Born 26 Jan 1799 in Paris, France
    http://sfabel.tripod.com/mathematik/database/Clapeyron.html
    Benoit Paul Emile Clapeyron
    Born: 26 Jan 1799 in Paris, France
    Died: 28 Jan 1864 in Paris, France
    Show birthplace location Previous (Chronologically) Next Biographies Index
    Previous
    (Alphabetically) Next Welcome page Emile Clapeyron Sadi Carnot 's ideas on heat analytically, with the help of graphical representations, in 1834. This work had important influences on Thomson and Clausius He was also interested in railway engineering and designed steam locomotives. The Clapeyron relation, a differential equation which determines the heat of vaporisation of a liquid, is named after him. References (5 books/articles) References elsewhere in this archive: Rue Clapeyron is in the 8th Arrondissement in Paris. You can see a list of Paris streets named after mathematicians in our archive. Previous (Chronologically) Next Biographies Index
    Previous
    (Alphabetically) Next Welcome page
    History Topics Index
    Famous curves index ... Search Suggestions JOC/EFR December 1996

    7. WIEM: Clapeyron Benoit Pierre Emile
    clapeyron Benoit Pierre emile (17991864), francuski fizyk, inynier i konstruktor (maszyn parowych i konstrukcji stalowych). Zajmowa si
    http://www.encyklopedia.pl/wiem/00094b.html
    wiem.onet.pl napisz do nas losuj: has³a multimedia Fizyka, Francja
    Clapeyron Benoit Pierre Emile widok strony
    znajd¼ podobne

    poka¿ powi±zane
    Clapeyron Benoit Pierre Emile (1799-1864), francuski fizyk, in¿ynier i konstruktor (maszyn parowych i konstrukcji stalowych). Zajmowa³ siê termodynamik±teori± sprê¿ysto¶ci Nada³ rozg³os pracom N.L.S. Carnota . Odkry³ zale¿no¶æ temperatury przej¶æ fazowych (topnienia, wrzenia) od ci¶nienia. By³ twórc± równania stanu gazu doskona³ego Zobacz równie¿ Clapeyrona równanie Powi±zania Carnot Sadi Nicolaus Leonard wiêcej zobacz wszystkie serwisy do góry Encyklopedia zosta³a opracowana na podstawie Popularnej Encyklopedii Powszechnej Wydawnictwa Fogra

    8. Clapeyron
    Benoit Paul emile clapeyron. Born 26 emile clapeyron was educated atthe Ecole Polytechnique from which he graduated in 1818. He then
    http://www-gap.dcs.st-and.ac.uk/~history/Mathematicians/Clapeyron.html
    Benoit Paul Emile Clapeyron
    Born: 26 Feb 1799 in Paris, France
    Died: 28 Jan 1864 in Paris, France
    Click the picture above
    to see a larger version Show birthplace location Previous (Chronologically) Next Biographies Index Previous (Alphabetically) Next Main index
    Emile Clapeyron was educated at the Ecole Polytechnique from which he graduated in 1818. He then went to the Ecole des Mines where he trained as an engineer along with his friend Gabriel Clapeyron and went together to Russia in 1820. Alexander I of Russia had set up a team of engineers to improve the roads and bridges of Russia and he turned to France to provide the expertise in teaching and leading the newly formed teams. Clapeyron and went to St Petersburg where the Ecole des Travaux Publics had been set up and these they taught both pure and applied mathematics. They both also engaged in leading the construction work as well as lecturing. Both Clapeyron and remained in Russia for 10 years. During that time they published joint mathematical and engineering work in a number of journals. They left following the revolution of 1830 when their political views made them feel unwelcome. They returned to France at a time when railways were just beginning to be constructed but the early ventures had been economic failures. They had the vision to see the great future in railways and money began to go into a study of engineering problems associated with their development. Clapeyron proposed a railway line from Paris to St Germain and sought funding for the project. However, before funding was obtained he was offered a chair at the Ecole des Mineurs in St Etienne. In 1835 the construction of the line from Paris to St Germain was authorised and Clapeyron and

    9. Clapeyron
    Biography of emile clapeyron (17991864) Benoit Paul emile clapeyron. Born 26 Feb 1799 in Paris, France
    http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Clapeyron.html
    Benoit Paul Emile Clapeyron
    Born: 26 Feb 1799 in Paris, France
    Died: 28 Jan 1864 in Paris, France
    Click the picture above
    to see a larger version Show birthplace location Previous (Chronologically) Next Biographies Index Previous (Alphabetically) Next Main index
    Emile Clapeyron was educated at the Ecole Polytechnique from which he graduated in 1818. He then went to the Ecole des Mines where he trained as an engineer along with his friend Gabriel Clapeyron and went together to Russia in 1820. Alexander I of Russia had set up a team of engineers to improve the roads and bridges of Russia and he turned to France to provide the expertise in teaching and leading the newly formed teams. Clapeyron and went to St Petersburg where the Ecole des Travaux Publics had been set up and these they taught both pure and applied mathematics. They both also engaged in leading the construction work as well as lecturing. Both Clapeyron and remained in Russia for 10 years. During that time they published joint mathematical and engineering work in a number of journals. They left following the revolution of 1830 when their political views made them feel unwelcome. They returned to France at a time when railways were just beginning to be constructed but the early ventures had been economic failures. They had the vision to see the great future in railways and money began to go into a study of engineering problems associated with their development. Clapeyron proposed a railway line from Paris to St Germain and sought funding for the project. However, before funding was obtained he was offered a chair at the Ecole des Mineurs in St Etienne. In 1835 the construction of the line from Paris to St Germain was authorised and Clapeyron and

    10. Benoît-Paul-Emile CLAPEYRON
    Translate this page et suiv. Benoît-Paul-emile clapeyron (1799-1864). clapeyron (Benoît-Paul-emile)est né à Paris, le 26 février 1799. Entré à
    http://www.annales.org/archives/x/clapeyron.html
    Publié dans le LIVRE DU CENTENAIRE (Ecole Polytechnique), 1897, Gauthier-Villars et fils, TOME I, pages 194 et suiv.
    Benoît-Paul-Emile CLAPEYRON (1799-1864)
    CLAPEYRON (Benoît-Paul-Emile) est né à Paris, le 26 février 1799. Entré à l'Ecole Polytechnique en 1816, il en sortit, en 1818, comme élève-ingénieur des Mines. A sa sortie de l'École d'application, en 1820, il partit pour la Russie, en compagnie de son antique et ami Lamé , dont il ne cessa d'être le fidèle collaborateur pendant sa longue et active carrière. Les deux jeunes ingénieurs étaient appelés par le gouvernement russe, pour importer et développer, dans ce pays encore neuf, la mécanique théorique et pratique, dont les progrès merveilleux excitaient l'émulation de tous les peuples civilisés. Indépendamment de nombreux travaux de construction, Clapeyron et Lamé furent chargés de l'enseignement des mathématiques pures et appliquées à l'École des Travaux publics de Saint-Pétersbourg. Ils demeurèrent dix années en Russie. Lorsque, en 1830, Clapeyron rentra en France, les esprits étaient en pleine effervescence. La locomotive la Fusée , construite par Stephenson, venait de remporter le prix du concours de Rainhill, et les perspectives les plus brillantes s'ouvraient à l'industrie des chemins de fer. Clapeyron n'hésita point à prendre une part active à ce mouvement. Il fut un des principaux promoteurs du railway de Paris à Saint-Germain; il en rédigea les projets et en dirigea la construction. Il fut aussi ingénieur du chemin de fer de Versailles (rive droite), et s'occupa principalement de la construction des machines locomotives. A cette époque, les lois qui régissent les mouvements de ces machines étaient encore mal connues : l'illustre Robert Stephenson, consulté par Emile Pereire, n'osa pas s'engager à fournir des locomotives, qui pussent franchir les rampes de 5 millimètres sur 18 kilomètres de développement que présente la ligne de Versailles. Clapeyron ne recula pas devant ce problème, et les machines furent construites, sur ses plans, par Sharp et Roberts.

    11. Ingenieurs Du Corps Des Mines
    Benoit Paul emile clapeyron (1799-1864); Edmond BOUR (1832-1866);
    http://www.annales.org/archives/
    Le corps des ingénieurs des mines au XIXème et au XXème siècles
    Mise à jour : 19 mars 2003 Cette page n'est pas figée ! Elle évolue grâce aux informations glanées, et aux contributions des lecteurs. Faites part de vos remarques et envoyez des éléments biographiques à Robert.Mahl@ensmp.fr
    Ouvrages de synthèse sur le corps des mines et l'Ecole des mines de Paris au XIXème siècle
    Les principaux textes législatifs ou règlementaires régissant le corps et le service des mines au XIXeme siècle

    12. Benoit Paul Emile Clapeyron - Anagrams
    Rearranging the letters of benoit paul emile clapeyron gives 'No, I beatpeople numerically!'! Find out for FREE what is hidden in YOUR name!
    http://www.anagramgenius.com/archive/benoit.html
    Rearranging the letters of 'Benoit Paul Emile Clapeyron' (Mathematician) gives:
    No, I beat people numerically! (by Mike Mesterton-Gibbons by hand)
    (FREE!)
    Download FREE anagram-generating software for your Windows computer
    Instructions for linking to this page!
    Learn about the Anagram Genius software (Windows/MacOS)
    Search the Archive
    Add YOUR anagrams to the Archive!
    League table of top contributors
    Find anagram aliases of benoit paul emile clapeyron (or any other text)!
    Find gold service anagrams of benoit paul emile clapeyron (or any other text)!
    Anagram Genius Archive Main Index
    Anagram Gems Mailing List
    Anagram Genius Archive France Index
    www.anagramgenius.com home page
    Crossword Maestro for Windows . The world's first expert system for solving crossword clues! Click here for more information or to download. William Tunstall-Pedoe . See this page for other points concerning benoit paul emile clapeyron.

    13. Benoît-Paul-Emile CLAPEYRON
    Publié dans le LIVRE DU CENTENAIRE (Ecole Polytechnique), 1897, GauthierVillars et fils, TOME I, pages 194 et suiv. Benoît-Paul-emile clapeyron (1799-1864) clapeyron (Benoît-Paul-emile) est né à Paris, le 26 février 1799.
    http://www.annales.com/archives/x/clapeyron.html
    Publié dans le LIVRE DU CENTENAIRE (Ecole Polytechnique), 1897, Gauthier-Villars et fils, TOME I, pages 194 et suiv.
    Benoît-Paul-Emile CLAPEYRON (1799-1864)
    CLAPEYRON (Benoît-Paul-Emile) est né à Paris, le 26 février 1799. Entré à l'Ecole Polytechnique en 1816, il en sortit, en 1818, comme élève-ingénieur des Mines. A sa sortie de l'École d'application, en 1820, il partit pour la Russie, en compagnie de son antique et ami Lamé , dont il ne cessa d'être le fidèle collaborateur pendant sa longue et active carrière. Les deux jeunes ingénieurs étaient appelés par le gouvernement russe, pour importer et développer, dans ce pays encore neuf, la mécanique théorique et pratique, dont les progrès merveilleux excitaient l'émulation de tous les peuples civilisés. Indépendamment de nombreux travaux de construction, Clapeyron et Lamé furent chargés de l'enseignement des mathématiques pures et appliquées à l'École des Travaux publics de Saint-Pétersbourg. Ils demeurèrent dix années en Russie. Lorsque, en 1830, Clapeyron rentra en France, les esprits étaient en pleine effervescence. La locomotive la Fusée , construite par Stephenson, venait de remporter le prix du concours de Rainhill, et les perspectives les plus brillantes s'ouvraient à l'industrie des chemins de fer. Clapeyron n'hésita point à prendre une part active à ce mouvement. Il fut un des principaux promoteurs du railway de Paris à Saint-Germain; il en rédigea les projets et en dirigea la construction. Il fut aussi ingénieur du chemin de fer de Versailles (rive droite), et s'occupa principalement de la construction des machines locomotives. A cette époque, les lois qui régissent les mouvements de ces machines étaient encore mal connues : l'illustre Robert Stephenson, consulté par Emile Pereire, n'osa pas s'engager à fournir des locomotives, qui pussent franchir les rampes de 5 millimètres sur 18 kilomètres de développement que présente la ligne de Versailles. Clapeyron ne recula pas devant ce problème, et les machines furent construites, sur ses plans, par Sharp et Roberts.

    14. Benoit Paul Emile Clapeyron: A Short Bibliographical Sketch
    Table of Contents. Abstract Volume 5 Issue 2 (2000) pp 8387 DOI 10.1007/s00897000370a.Benoit Paul emile clapeyron A Short Bibliographical Sketch. Jaime Wisniak.
    http://chemed.boisestate.edu/bibs/0005002/00050083.htm
    TCE
    The Chemical Educator
    ISSN: 1430-4171 (electronic version) Table of Contents Abstract Volume 5 Issue 2 (2000) pp
    DOI 10.1007/s00897
    Benoit Paul Emile Clapeyron: A Short Bibliographical Sketch
    Jaime Wisniak Department of Chemical Engineering, Ben-Gurion University of the Negev, Beer-Sheva, Israel Published online: 2 April 2000 Abstract. Clapeyron is well known in thermodynamics through the Clausius-Clapeyron equation that describes the variation of the vapor pressure with temperature; few are aware that he made his career as a railroad engineer and locomotive designer. Here we give a description of his life and his scientific and professional achievements as an engineer in a turbulent epoch in the history of France and Russia. It is shown that Clapeyron was able to develop his equation without making use of the second law and the concepts of absolute temperature and entropy, ideas unknown in his time. Key Words: Chemistry and History;

    15. Great Physicists
    1832); Cavendish, Henry (1731 1810); Chandrasekhar, Subrahmanyan(1910 - 1995); clapeyron, Benoit Paul emile (1799 - 1864); Clausius
    http://physics.hallym.ac.kr/reference/physicist/physicist.html
    Great Physicists
    Most of the following materials are from History of Mathematics Archive , St Andrews Univ., UK. For links that don't work, go to the original places above.
  • Newton, Sir Isaac (1643 - 1727)
  • Maxwell, James Clerk (1831 - 1879)
  • Franklin, Benjamin : to Franklin Institute Science Museum at Philadelphia
  • Geiger, Hans
  • Gell-Mann, Murray
  • Goudsmit, Samuel Abraham (1902-1978)
  • Halley, Edmond (1656 - 1742) ...
  • Millikan, Robert : Caltech Photo Archive
  • Minkowski, Hermann (1864 - 1909)
  • von Neumann, John (1902 - 1957) here
  • Ohm, Georg Simon (1789 - 1854) ...
  • Noted Figures in Physics, Engineering and Astronomy
  • 16. Clapeyron Portrait
    Portrait of emile clapeyron emile clapeyron. JOC/EFR August 2001
    http://www-history.mcs.st-and.ac.uk/history/PictDisplay/Clapeyron.html
    Emile Clapeyron
    JOC/EFR August 2001 The URL of this page is:
    http://www-history.mcs.st-andrews.ac.uk/history/PictDisplay/Clapeyron.html

    17. Clapeyron
    clapeyron. Benoit Paul emile clapeyron. Born France. emile clapeyron waseducated at the Ecole Polytechnique from which he graduated in 1818.
    http://www.vivoscuola.it/us/rsigpp3202/umidita/copie/clapeyron.htm
    clapeyron
    Benoit Paul Emile Clapeyron
    Born: 26 Feb 1799 in Paris, France
    Died: 28 Jan 1864 in Paris, France
    Emile Clapeyron was educated at the Ecole Polytechnique from which he graduated in 1818. He then went to the Ecole des Mines where he trained as an engineer along with his friend Gabriel Lamé. Clapeyron and Lamé went together to Russia in 1820. Alexander I of Russia had set up a team of engineers to improve the roads and bridges of Russia and he turned to France to provide the expertise in teaching and leading the newly formed teams. Clapeyron and Lamé went to St Petersburg where the Ecole des Travaux Publics had been set up and these they taught both pure and applied mathematics. They both also engaged in leading the construction work as well as lecturing. Both Clapeyron and Lamé remained in Russia for 10 years. During that time they published joint mathematical and engineering work in a number of journals. They left following the revolution of 1830 when their political views made them feel unwelcome. They returned to France at a time when railways were just beginning to be constructed but the early ventures had been economic failures. They had the vision to see the great future in railways and money began to go into a study of engineering problems associated with their development. Clapeyron proposed a railway line from Paris to St Germain and sought funding for the project. However, before funding was obtained he was offered a chair at the Ecole des Mineurs in St Etienne. In 1835 the construction of the line from Paris to St Germain was authorised and Clapeyron and Lamé were put in charge of the project. Lamé was offered the chair of physics at the Ecole Polytechnique shortly after they began their work and Clapeyron was left to head the venture.

    18. The Science Bookstore - Chronology
    clapeyron, emile Died 1/26/1799, 1799 AD, 1834 AD, clapeyron, B. BenoitPierreclapeyron presents a formulation of the second law of thermodynamics.
    http://www.thesciencebookstore.com/chron.asp?searchstring=Clapeyron

    19. The Science Bookstore - Chronology
    1799 AD, de Laplace, P. PierreSimon de Laplace begins publication of CelestialMechanics finished in 1825. clapeyron, emile Died 1/26/1799, 1799 AD,
    http://www.thesciencebookstore.com/chron.asp?pg=12

    20. Clapeyron
    Translate this page Histoire de la chimie. emile clapeyron. 1799-1884. Physicien français.Un des fondateurs de la thermodynamique notamment par son mémoire
    http://www.sciences-en-ligne.com/momo/lic/chimie/hist_chi/biographies/c_f/Biog_C
    Histoire de la chimie Emile Clapeyron Physicien français. Un des fondateurs de la thermodynamique notamment par son mémoire sur la Force motrice de la chaleur de 1834 dans lequel il commente et développe les travaux de S. Carnot . Il énonça la loi des gaz parfaits.

    A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z  

    Page 1     1-20 of 88    1  | 2  | 3  | 4  | 5  | Next 20

    free hit counter