The year 1784 in science and technology involved some significant events.

• September 10 – Edward Pigott identifies the variable star Eta Aquilae from York, England.
• October 19 – John Goodricke begins his observations of the variable star Delta Cephei from York.

• Publication of the Annals of Agriculture edited by Arthur Young begins in Great Britain.
• Peter Simon Pallas begins publication of Flora Rossica, the first Flora of Russia.

• L'Abbé René Just Haüy states the geometrical law of crystallization.^[1]
• Antoine Lavoisier pioneers stoichiometry.
• Citric acid is first isolated by Carl Wilhelm Scheele, who crystallizes it from lemon juice.^[2][3]
• Cholesterol is isolated.

• Publication of David Bourgeois' Recherches sur l'art de voler, depuis la plus haute antiquité jusque'a ce jour in Paris, the earliest work on the history of flight.

• Carl Friedrich Gauss, at the age of seven, pioneers the field of summation with the formula summing 1:n as (n(n+1))/2.

• 11 February – Royal College of Surgeons in Ireland chartered.
• 12 March — Appointment of the French Royal Commission on Animal Magnetism (the Commission's Report was presented to King Louis XVI on 11 August 1784).
• Madame du Coudray, pioneer of modern midwifery in France, retires.
• Benjamin Franklin makes the first known specific reference (in a letter) to the wearing of bifocal spectacles.^[4]
• John Hunter first describes the condition phlebitis.

• The first description of a Pterodactylus fossil is made by Cosimo Alessandro Collini, although he is unable to determine what kind of creature it is.^[5][6][7]

• January 15 – Henry Cavendish's paper to the Royal Society of London, Experiments on Air, reveals the composition of water.^[8]
• Jean-Paul Marat publishes Notions élémentaires d'optique (Elementary Notions of Optics)

• William Roy measures the baseline for the Anglo-French Survey (1784–1790) linking the observatories of Paris and Greenwich. The measurement is accurate to within a few inches in a distance of over 280,000 ft., an unprecedented accuracy for this time. Roy is awarded the Copley Medal in the following year.^[9]

• April 28 – James Watt receives a British patent for his parallel motion and other improvements to the steam engine.^[10]
• June 4 – Élisabeth Thible becomes the first woman passenger in a hot-air balloon, at Lyon, France.^[11]
• August 21 – Joseph Bramah receives his first lock patent in London.
• Rev. Dr. Edmund Cartwright designs his first power loom in England.
• Henry Cort of Funtley, England, applies the coal-fired reverbatory furnace to the puddling process for conversion of cast to wrought iron.^[12]

• Copley Medal: Edward Waring^[13]

• March 12 – William Buckland, English geologist and paleontologist (died 1856)
• June 17 – Andrew Crosse, English 'gentleman scientist', pioneer experimenter in electricity (died 1855)
• July 22 – Friedrich Bessel, German mathematician (died 1846)
• September 1 – Thomas Frederick Colby, English cartographer (died 1852)

• May 12 – Abraham Trembley, Genevan naturalist (born 1710)
• September 1 – Jean-François Séguier, French astronomer and botanist (born 1703)
• September 4 – César-François Cassini de Thury, French astronomer (born 1714)