law) in computer engineering: any sufficiently large problem can be efficiently parallelized. Coined by John Gustafson in 1988. H–K Haber's rule is a mathematical statement relating the concentration of a poisonous gas and how long it must be breathed to result in death. Hagen–Poiseuille law: a physical law that gives the pressure drop in an incompressible and Newtonian fluid in laminar flow flowing through a long cylindrical pipe of constant cross section. Named after Gotthilf Hagen and Jean Poiseuille. Haitz's law is an observation and forecast about the steady improvement, over many years, of light-emitting diodes (LEDs). Hamilton's principle: the dynamics of a physical system is determined by a variational problem for a functional based on a single function, the Lagrangian, which contains all physical information concerning the system and the forces acting on it. Named after William Rowan Hamilton. Hanlon's razor is a corollary of Finagle's law, named in allusion to Occam's razor, normally taking the form "Never attribute to malice that which can be adequately explained by stupidity." As with Finagle, possibly not strictly eponymous. Alternatively, "Do not invoke conspiracy as explanation when ignorance and incompetence will suffice, as conspiracy implies intelligence." Hartley's law is a way to quantify information and its line rate in an analog communications channel. Named for Ralph Hartley (1888–1970). Hasse principle is the idea that one can find an integer solution to an equation by using the Chinese remainder theorem to piece together solutions modulo powers of each different prime number. Named after Helmut Hasse. Hauser's law is an empirical observation about U.S. tax receipts as a percentage of GDP, theorized to be a natural equilibrium. Heaps' law describes the number of distinct words in a document (or set of documents) as a function of the document length. Hebb's law: "Neurons that fire together wire together." Heisenberg's uncertainty principle: one cannot measure values (with arbitrary precision) of certain conjugate quantities, which are pairs of observables of a single elementary particle. The most familiar of these pairs is position and momentum. Henry's law: The mass of a gas that dissolves in a definite volume of liquid is directly proportional to the pressure of the gas provided the gas does not react with the solvent. Hess's law, in physical chemistry: the total enthalpy change during the complete course of a reaction is the same whether the reaction is made in one step or in several