LOGARITHMIC SLIDE RULES. 105 
root of numbers with an uneven number of integers and the right hand 
scale for numbers with an even number of integers. 
To square a number the converse of this procedure must be adopted 
and the answer will have an odd or even number of integers according 
as it is found on the left or right hand upper scale on the rule. If any 
difficulty is experienced in remembering any of these methods, a trial, 
using well known numbers, will very soon put the operator right. 
To take the cube root of a number place the cursor over the 
number on the upper scale of the rule, say 36 {i.e. on the right hand 
scale, see Fig. 12) then move the slide to the right until some number 
on the upper scale of the slide coincides with the cursor at the same 
time that the left hand 1 on the lower scale of the slide is above 
the same number on the rule. This is the number required. Thus 
the cube root of 36 is found to be 3*3. 
To find the area of a circle, place the long index line marked at 7854 
on the right hand upper scale of the slide under the right hand 1 on the 
rule. Place the cursor over the diameter of the circle on the lower 
scale of the rule and read off the area in square inches under the 
cursor on the upper scale of the slide . 
E.g. Find cubic capacity of a tube 103 inches long and 6 inches 
in diameter. 
Area of the circle = 28*25 square inches. (See Fig. 13). 
Then using the lower scales 28*25 x 103 = 2910 cubic inches. 
It is often required to solve triangles, as for instance in Siege 
Artillery practice where the base and all three angles are known. 
Sin A Sin B 
Reverse the slide so that the scale of sines is under the upper scale 
of the rule. Place Sin A under the length of a and over Sin B will 
be found b (see Fig. 14). 
300 _ x 1255 
Sin 13° Sin 70° Sin 70° 
Only a few examples illustrating the use of the logarithmic slide rules 
have been given, but it is hoped that they are sufficient to show how 
to manipulate it and also to prove how useful the knowledge of the 
instrument ought to be to those officers whose work often entails 
making calculations. It is particularly useful in working out the 
same example with one factor continually changing, such as the angle 
of sight for different ranges from a gun mounted at a particular height 
above sea level. It is also most convenient to use it for seeing at a 
glance the percentage of marks gained in an examination by each 
member of a class when the maximum is not exactly 100. 
The slide rule can be easily applied for the conversion of scales in 
connection with military topography work, while for examination 
purposes it is distinctly valuable to check roughly and quickly any 
calculations for which, owing to the want of more than three figures, 
its use has been prohibited in the first instance. 
