the first of the woodstocks, the hp21 replaced the hp35
as the basic scientific model in 1975.
like all the woostocks, it has a nice shape and size. it starts up
displaying two decimal places by default which can be changed up to 8+2. a
switch selects between degrees or radians. notice that the shifted
functions are written on the key slope rather than on the faceplate and
that the four basic operators are on the left side of the keyboard. later
all calculators gravitated towards the, now normal, +  * / on the right.
congratulations!
your hp21 is another professionalquality pocket instrument from the
hewlettpackard line of calculators  calculators whose durability, small
size, and ease of operation have made them the choice for use by climbers
on the frigid crags of mt. everest and astronauts orbiting the vast depths
of space, as well as doctors, engineers, scientists, and other people who
require instant, accurate answers to complex and highly technical
problems. you're in good company with hp!
can you calculate?
the manual contains many fascinating example problems. here's one of
them
having lost most of his equipment in a blinding
snowstorm, ace explorer buford eugobanks is using an ordinary barometer as
an altimeter. after measuring the sea level pressure (30" of mercury)
he climbs until the barometer indicates 9.4" mercury. although the
exact relationship of pressure and altitude is a function of many factors,
eugobanks knows than an approximation is given by the formula: alt (feet)
= 25,000 ln(30/pressure).
where is buford eugobanks? 
