LOGARITHMIC SLIDE RULES. 
103 
on the slide must be placed over the 24 on the rule and under the 83 
on the slide will be found 1992, and there will be four figures in the 
answer according to the rule. If we had to multiply 2*4 x 8*3 the 
procedure would be exactly the same, but there would be only two 
integers in the answer, which would, therefore, read 19*92. i? ] 
Again, 246 x 357 = 87800 by slide rule. (See Fig. 2.) The exact 
answer is 87822, but in many cases 87800 would be sufficiently 
accurate. 
Another method for locating the decimal point is adopted sometimes 
as follows :— 
To multiply 33 by 245, divide the first factor mentally by 10 and the 
second by 100, and we get 3*3 x 2*45 = a product 8080. It is seen 
at a glance the answer is 8*080 ; then multiply by the 1000 ( i.e . 10 
and 100) that the factors had been divided by, and the correct answer 
8080 is obtained. The absolutely correct answer is 8085, but as I 
have already explained the fourth figure can only be an approximation. 
DIVISION. 
Proceed as in vulgar fractions. 
83 _ x 
24 1 
Using the lower scales (see Fig. 3) put the cursor over the 
24 on the rule, run the 83 on the slide over this and then over the 1 on 
the rule will be found 346 on the slide . The rule for the number of 
integers is “If the slide is out to the right, subtract the number of 
integers in denominator from that in numerator, the remainder will 
give number of integers in answer. If the slide is out to the left 1 
must be added to this,” so that in the above example the answer is 
3*46. In simple cases such as this the position of the decimal point 
can be fixed by mere inspection. 
PROPORTION. 
u 
86 
Using the upper scales put 86 on the slide under 14 on 
the rule, (see Fig. 4) then above 62 on the slide will be found^l‘01 
(the cursor being used to facilitate the reading). > % 
Using the lower scales the answer could be read more accurately, 
i.e., 10*45, but so often in proportion the lower scales cannot be used 
owing to the scales being single, that it is better as a rule to keep to 
the upper scales for this calculation. 
TRIGONOMETRY. 
Given angle of descent 51 minutes, find slope of descent. 
For any angle under 5° 45' the sines and tangents may be taken to 
be the same. 
Turn the rule over and pull slide out to the right. A single line or 
index in the upper side of the notch of the rule will be observed 
pointing to the scale of sines. Move the slide till the 7° is under the 
index. (See Fig. 5). It is easy to remember that tan 7° or sin 7° 
= approximately = |; now turn the rule over (see Fig. 6) and 
the left hand 1 on the top scale of the slide will be seen under 8*2, 
