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In marine navigation a "hack watch" was used for observations topside so
the ship's chronometers could remain undisturbed below deck. There was
no requirement that the watch be settable to the second. It was
sufficient to record how much the hack watch was fast or slow on the
chronometers.

Shadwell ("Notes on the management of chronometers," 1861) uses the term
"assistant-watch":
"The practice, therefore, of taking a chronometer on deck or on shore,
for the purpose of observation, should never be resorted to; a
pocket-chronometer, or a good pocket-watch with a second-hand, should on
these occasions be used as an assistant."
https://books.google.com/books?id=1h0PAAAAYAAJ&pg=PA36&focus=viewport&dq=editions:vxpymHQeOAYC

Lecky ("Wrinkles in Practical Navigation," 1884):
"It is well to have a fourth chronometer in the second officer's room,
as a hack-watch for general use among the officers."
https://books.google.com/books?id=BzIAAAAAQAAJ&pg=PA26&focus=viewport

Martin ("A Treatise on Navigation and Nautical Astronomy," 1899):
"The times of making all observations are noted by a smaller portable
chronometer termed a hack watch, which is usually constructed to beat 5
times in 2 seconds (box chronometers generally beat half-seconds), and
which, being more liable to change from being carried about than the box
chronometers, is compared with their indications at as near the time of
observation as possible."
https://books.google.com/books?id=NP0nAAAAYAAJ&pg=PA278&focus=viewport

Bowditch ("American Practical Navigator," 1914):
"To avoid derangement, the chronometers should never be removed from the
permanent box in which they are kept on shipboard. When it is desired to
mark a certain instant of time, as for an astronomical observation or
for obtaining the chronometer error by signal, the time is marked by a
hack (an inferior chronometer used for this purpose only), or by a
comparing watch."
https://books.google.com/books?id=FvAqAAAAYAAJ&jtp=100

Muir ("A Treatise on Navigation and Nautical Astronomy," 1918):
"It has already been said that the chronometers should not be subject
even to occasional removal. This is true, and in order to get the
chronometer times of certain desired instants, as the instant of receipt
of the noon signal, or the drop of a time ball, etc., use must be made
of a less valuable time piece known as the hack, an inferior grade of
chronometer, or of a comparing watch."
https://books.google.com/books?id=h6xEAAAAIAAJ&pg=PA337&focus=viewport

A modern equivalent of a hack watch is the ATTU (airborne time transfer
unit) of the B-2 bomber. Its AINS (astro inertial navigation system) has
an internal chronometer, but before flight it must be synchronized with
UTC. The human eye, ear, and hand are not accurate enough. It would be
inconvenient to wheel a rack mounted time standard out to the plane
before each flight. Possibly the rubidium oscillator would not enjoy the
disturbance either.

The solution is to transfer the time via an ATTU. You install three 9v
batteries, connect it to the time standard, and by manipulating a few
switches, "hack" the ATTU. It is then carried to the B-2 and installed
before engine start. When avionics are powered up, the AINS obtains UTC
from the ATTU. The latter is not needed for the rest of the flight.

Incidentally, the AINS has a catalog of 61 stars. I don't know their
identities. Nor did I ever find time to inquire about how the AINS knows
the current value of UT1-UTC. I believe it's included in the mission
data — the waypoints, targets, etc. that are the objectives of the
flight — which are loaded into the flight management system.

   

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