_Magnifying Cursors_ are of assistance in reading the scales, and in a good and direct light are very helpful. In one form an ordinary lens is carried by two light arms hinged to the upper and lower edges of the cursor, so that it can be folded down to the face of the rule when not in use. A more compact form, shown in Fig. 8, consists of a strip of plano-convex glass, on the under-side of which is the hair-line. In a cursor made by Nestler of Lahr, the plano-convex strip is fixed on the ordinary cursor. The magnifying power is about 2, so that a 5 in. rule, having the same number of graduations as a 10 in. rule, can be read with equal facility, by the aid of this cursor. The Digit-registering Cursor, supplied by Mr. A. W. Faber, London, and shown in Fig. 9, has a semicircular scale running from 0 at the centre upward to −6 and downward to +6. A small finger enables the operator to register the number of digits to be added or subtracted at the end of a lengthy operation, as explained at page 28. MULTIPLICATION. In the preliminary notes it was shown that by mechanically adding two lengths representing the logarithms of two numbers, we can obtain the _product_ of these numbers; while by subtracting one log. length from another, the number represented by the latter is divided by the number represented by the former. Hence, using the C and D scales, we have the RULE FOR MULTIPLICATION.—_Set the index of the C scale to one of the factors on D, and under the other factor on C, find the product on D._ [Illustration: FIG. 10.] Thus, to find the product of 2 × 4, the slide is moved to the right until the left index (1) of C is brought over 2 on D, when under the other factor (4) on C, is found the required product (8) on D. Following along the slide, to the right, we find that beyond 5 on C (giving 10 on D), we have no scale below the projecting slide (Fig. 10). If we imagine the D scale prolonged to the right, we should have a repetition of the earlier portion, but, as with the two parts of the A scales, the repeated portion would be of tenfold value, and 10 on C would agree with 20 on the prolonged D scale. We turn this fact to account by moving the slide to the left until 10 on C agrees with 2 on D, and we can then read off such results as 2 × 6 = 12; 2 × 8 = 16, etc., remembering that as the scale is now of tenfold value, there will be two figures in the result. Hence, for those who prefer rules, we have the RULE FOR THE NUMBER OF DIGITS IN A PRODUCT.—_If the product is read with the slide projecting to the_ LEFT, ADD THE NUMBER OF THE DIGITS IN THE TWO FACTORS; _if read with the slide to the_ RIGHT, _deduct 1 from this sum_. EX.—25 × 70 = 1750. The product is found with the slide projecting to the _left_, so the number of digits in the product = 2 + 2 = 4. EX.—3·6 × 25 = 90. The slide projects to the _right_, and the number of digits in the product is therefore 1 + 2 − 1 = 2.