BY J. C. BRUNNICH, F.I1.C. 39 
ally used decimal or common logarithms. Already in 
1657 Seth Partridge constructed a logarithmic slide rule 
with Gunter’s scales, which is really the forerunner of all 
the sliderules in use at the present day. Although England 
is the home of the original inventors, the use of sliderules 
made very little progress in the country, and only within 
recent years more attention has been given to the little 
instrument, which is becoming of more general use. In 
France, Germany and other European countries sliderules 
are very much more extensively used, and are not only 
used by nearly every scientist, but are found in the hands 
of every artisian, mechanic and engineer. The reason 
that sliderules are less used in England and its Colonies 
lies unquestionably in the fact that the ordinary worker 
on account of the complicated system of weight, measures 
and monies, is not so accustomed with the use of decimals, 
but does most of his calculations with vulgar fractions. 
The principles, a mechanical and mathematical one, 
on which the use of sliderules are based, are exceedingly 
simple and easily understood. We will first consider the 
mechanical principle, which is easily demonstrated by taking 
two ordinary scales divided into 10 equal parts, in contact 
with each other (Table VIII., Plate III), by now moving 
the lower scale until the zero falls below a certain number, 
for instance three, in the upper scale, we will find that every 
other number on the upper scale is equal to the sum of the 
coinciding number on the lower scale plus three. This 
gives a simple method of adding a number taken on one scale 
to any number on the other scale. Similarly substraction 
may be demonstrated by placing the number to be de- 
ducted underneath the number from which it has to be 
substracted, for instance, four from seven, and to read off 
the result of the substraction over zero of the lower scale on 
the upper scale, which equals three. For certain operations 
the lower scale may be inverted, and in this case we will 
find that the sum of all the coinciding figures on the two 
scales is constant, and as an example we find on our table 
that this sum of numbers is seven. We now see that with 
the slide inverted the sum of the numbers is constant, 
whereas with the slide direct the difference of numbers 
is constant. 
