Where does head collide?

In hydraulics, head is the mechanical energy carried by a fluid, expressed as the height of a liquid column that the energy could support. Daniel Bernoulli's 1738 Hydrodynamica established that a fluid's pressure, speed, and elevation trade off along a streamline, the relation later written as the Bernoulli equation, in which each energy term appears as a height and their sum is the total head. The referent here is an energy-per-weight quantity of a fluid, measured in metres or feet of liquid, and it is unrelated to the central word of a phrase in grammar or to the reading device of an abstract computing machine.[1]

Unit

The mechanical energy of a fluid per unit weight, given as a height of liquid column

Recovery means

Not applicable

Measured by

Read from a piezometer or manometer as the elevation to which the liquid rises, in metres or feet of fluid

In syntax, the head is the one word within a phrase that fixes the phrase's grammatical category and much of its behaviour, so that a noun heads a noun phrase and a verb heads a verb phrase. Leonard Bloomfield's 1933 Language introduced the term for the central member of an endocentric construction, the part the whole phrase can stand in for, and Zwicky's 1985 Heads examined the several distinct criteria, among them category determination, agreement, and government, that the notion is asked to satisfy at once. The referent is a relation between a word and the phrase built around it, identified by grammatical tests rather than any physical scale, and it has nothing in common with a fluid's stored energy or the scanning part of a computing machine.[2][3]

Unit

The single constituent within a phrase that determines the phrase's category and properties

Recovery means

Not applicable

Measured by

Identified by constituency and distribution tests: the element the whole phrase can substitute for and whose category the phrase shares

In the theory of computation, the head is the part of a Turing machine that scans the tape, reading the symbol in the current cell, writing a symbol there, and then moving one cell to the left or right. Alan Turing's 1936 On Computable Numbers defined this abstract machine, in which a single scanning element examines and alters one square of an unbounded tape according to the machine's state, and later automata theory named that element the head. The referent is a formal reading and writing device inside a mathematical model of computation, governed by a transition rule, and it bears no relation to a fluid's energy expressed as height or to the central word of a grammatical phrase.[4]

Unit

The single scanning element that reads and writes one tape cell at a time

Recovery means

Not applicable

Measured by

Described by the transition function that, given the current state and scanned symbol, sets the written symbol, the next state, and the head's move left or right