58 Fuller's Calculating Slide-Rule. 



may be completed by interposing in like manner the inter- 

 mediate numbers, and any others in the third place that the 

 length of scale will allow. Now, if with a pair of com- 

 passes the space spanned from the commencement of the 

 scale to any number be simply added on to some other 

 number, the resulting product will be found indicated at the 

 lower leg of the compass, and similarly if the space between 

 two numbers be measured, and an equal space be laid off 

 from the top of the scale, the resulting quotient will be also 

 shown at the lower point. A very few experiments with 

 this scale, however, will exhibit one defect, viz., that the 

 lower leg of the compass often falls altogether out of the 

 scale ; and hence the necessity of a double scale for practical 

 utility, such as is adopted in the common Carpenters Slide- 

 Rule* In this rule, which is usually about a foot long, the 

 results are obtained by placing scale against scale. There are, 

 in fact, four distinct scales, marked A, B, C, and D respec- 

 tively — the three former, which are alike, being double 

 logarithmic scales, and the fourth being a single logarithmic 

 scale exactly double the length of the others. This latter is 

 employed for finding, in conjunction with the other scales, 

 squares and square-roots. This slide-rule is well known, 

 and is, I believe, much used in England amongst carpenters, 

 mechanics, and others for rough calculations. But in this 

 colony I have not met with a single workman who under- 

 stood its use, and it is usually looked upon merely as a 

 useful adjunct to the foot-rule for the measurement of 

 inches. 



[Another form of logarithmic rule to which reference ought 

 to be made is the circular one, which, although not so well 

 known as the common slide-rule, possesses many advantages 

 over it. In the circular rule only a single scale is necessary; 

 the slide is dispensed with, and the operations are performed 

 by two hands or indices instead.-^] 



* An interesting account of the history and use of this Eule is to be 

 found in a pamphlet entitled " The Carpenter's Slide-Rule," published by 

 Messrs. John Eabone and Sons, of. Birmingham. 



f Since the reading of this paper, I have had an opportunity of examining a 

 most ingenious and portable form of the circular logarithmic scale under the 

 name of the " Cercle a Calcul." In size and general appearance this instru- 

 ment resembles a watch. It has two hands or indices, one fixed and the other 

 movable, so that they may be placed in any required relation to each other ; 

 and on the two faces are engraved the scales. One of these faces is movable 

 by means of a thumb-screw, such as is used in a keyless watch, whilst the 

 opposite face is fixed, and may be traversed by a needle on the same pivot 

 as, and with corresponding motions to, the needle (or movable index) on the 



