101 
LOGARITHMIC SLIDE RULES. 
thus shewing how the slope will be read off ; now move the slide out till 
the index in the notch at the back points to 51 minutes, and on looking 
at the front of the rule a slope of A- i s read off (see Fig. 7). 
For any angle greater than 5° 45', the left hand notch and index 
should be used in conjunction with the lower scales. Thus an angle 
of 15° 10' gives a slope of A 
For angles less than 34 minutes the slide must be removed from the 
rule turned over and inserted with the scale of sines immediately 
under the upper scale on the rule. Take 22 minutes for example; 
place the line that will be found just to the left of the 2° (called the 
minute index) under the 22 on the right upper scale on the rule, (the 
cursor is marking it in Fig. 8), then over the extreme left hand mark 
of the slide will be found 64, i.e. f tan 22' = *0064. In order to 
locate the decimal point, I find it convenient to paste a piece of 
paper on the back of the rule and write :— 
Sin 90° 
„ 5° 45' 
„ 34' 
„ 3-4' 
1 
*1 
•01 
•001 
Now replace the slide in its proper position and work out the 
following sum of proportion :— 
•0064 1 . 1 
1 x 156 
If any difficulty is experienced in knowing whether the denominator 
is 156, 15*6 or 1560, a very easy solution is found by noticing that 
•0064 •64 6-4 , . 1 
1 100 - 1000 - a ou 160 
To find the angle of descent from the slope of descent, or to find the 
range finding angle for any range from any given height the work is 
exactly the converse of the previous examples. 
To find the range-finding angle or angle of sight to a point at 1500 
yards from a gun mounted 130 feet above sea level. 
ISO 13 
- --- = ^ Move the slide until the 13 on the left hand upper 
4500 450 
scale of the slide comes under the 45 on the right hand scale of the 
rule. (See Fig. 9). At the back will be found the angle 1°39'. (See 
Fig, 10). 
To find the square root of a number, for example 73. Place the 
cursor over fiie 73 on the right hand upper scale on the rule. (See 
Fig. 11). Under the cursor on the lower scale of the rule will be found 
the answer 8*55. To find the square root of 7*3 the cursor must be 
placed over the 73 on the left hand upper scale on the rule , and the 
answer 2-7 will be found on the lower scale of the rule. Hence the 
rule is to use the left hand upper scale of the rule for finding the square 
