LOGARITHMIC SLIDE RULES. 
102 
A scale of inches on the bevelled edge. 
The upper scale on the rule 4 a . M 
Trl i Similar to each other. 
33 33 33 33 33 Sllue ) 
The lower scale on the slide 4 Similar to each other and twice 
„ ,, „ ,, ,, rule j the scale of the upper two. 
On the reverse of the slide will be found (Figs. 5 and 14) :— 
A scale of logarithmic sines (marked S at the right end). 
33 33 33 logarithms. 
33 33 3 i logarithmic tangents (marked Tab the right end). 
There are also on the reverse side two recesses or notches with 
index lines to be used in conjunction with these last three scales. 
On moving the slide out to the right when holding it the right side 
up, there will be seen a scale of inches in the recess in the centre of the 
rule and by combining this and the scales of inches on each edge of 
the rule anything up to 20 inches can be measured. 
A few words are necessary in order to explain the notation of the 
rule. In Fig. 1 the cursor will be seen cutting the upper and lower 
scales on the rule at 21 and 145 respectively, and it will be noticed 
that by moving it to the right until it reaches the 2 on the lower scale, 
any number with three figures can be read off: and an approximation 
even of a fourth can be made. Continuing to the right one finds the 
sub-divisions begin to get smaller and, therefore, are not numbered, 
and from 20 to 21 there are only five divisions instead of ten. 
Similarly from 40 to 41 there are only two divisions. Care must be 
taken when reading from the left end of the lower scale, where the 
cursor is in Fig. 1, not to forget that the number is 145 and not 45. 
It must also be understood that just as when working with 
logarithm tables the decimal place has to be located afterwards by 
inspection. 
The following are some of the calculations which continually occur 
in connection with gunnery and also other calculations. 
MUIiTIPIilCATIOM. 
Whenever possible use the lower and larger pair of scales. 
To multiply 24 by 37 ( i.e . to add the log of 24 to the log of 37) 
place the left hand 1 on the slide over the 24 on the rule (see Fig. 1) 
and under the 37 on the slide will be found on the rule the reading 
888. It is seen that it is something between 885 and 890 and the 
third figure is easily arrived at by noticing the last figure of each of 
the factors (i.e. the product of four and seven must end in eight). 
There is nothing on the rule to show whether the answer is 8*88, 88*8, 
888 or 8880. A simple rule will suffice. “ If in multiplying, the 
slide has been pulled out to the right, the number of integers in the 
answer will be one less than the sum of the integers in the two factors; 
if the slide has been moved to the left the sum of the integers in the 
answer will be equal to the sum of the integers in the two factors. 
For example 24 x 83. By proceeding as in the former example, 
the 83 on the slide will come outside the rule and so the right hand 51 
