formula log
loga^x = log loga + logx. These rules are manufactured by A.G. Thornton of
Manchester.

Many different forms of slide rules are now on the market. The handiest for
general use is the Gravet rule made by Tavernier-Gravet in Paris, according
to instructions of the mathematician V.M.A. Mannheim of the Ecole
Polytechnique in Paris. It contains at the back of the slide scales for the
logarithms of sines and tangents so arranged that they can be worked with
the scale on the front. An improved form is now made by Davis and Son of
Derby, who engrave the scales on white celluloid instead of on box-wood,
thus greatly facilitating the readings. These scales have the distance from
one to ten about twice that in fig. 4. Tavernier-Gravet makes them of that
size and longer, even 1/2 metre long. But they then become somewhat unwieldy,
though they allow of reading to more figures. To get a handy long scale
Professor G. Fuller has constructed a spiral slide rule drawn on a
cylinder, which admits of reading to three and four figures. The handiest
of all is perhaps the "Calculating Circle" by Boucher, made in the form of
a watch. For various purposes special adaptations of the slide rules are
met with--for instance, in various exposure meters for photographic
purposes. General Strachey introduced slide rules into the Meteorological
Office for performing special calculations. At some blast furnaces a slide
rule has been used for determining the amount of coke and flux required for
any weight of ore. Near the balance a large logarithmic scale is fixed with
a slide which has three indices only. A load of ore is put on the scales,
and the first index of the slide is put to the number giving the weight,
when the second and third point to the weights of coke and flux required.

By placing a number of slides side by side, drawn if need be to different
scales of length, more complicated calculations may be performed. It is
then convenient to make the scales circular. A number of rings or disks are
mounted side by side on a cylinder, each having on its rim a log-scale.

The "Callendar Cable Calculator," invented by Harold Hastings and
manufactured by Robert W. Paul, is of this kind. In it a number of disks
are mounted on a common shaft, on which each turns freely unless a button
is pressed down whereby the disk is clamped to the shaft. Another disk is
fixed to the shaft. In front of the disks lies a fixed zero line. Let all
disks be se