June 3, 1921] 



SCIENCE 



519 



able pointer (P), bearing a median bair line 

 tbroughout its length, is placed between the 

 two disks and consists of a strip of trans- 

 parent celluloid pivoted at one end on the 

 eyelet and extending with a sharply pointed 

 tip to the periphery of the larger disk. 



3. Scale A, marked on the acircular sheet 

 exetrnal to the circumference of the larger 

 disk, is comprised of 365 equidistant radial 

 lines numbered by tens from (index a) to 

 360, representing successive days. Every 

 seventh line if accented by a dot and num- 

 bered serially from (index a) to 52, indi- 

 cating weeks. An outer scale divides the 

 circumference into twelve equal parts, each in 

 turn divided into fourths, and is numbered 

 from to 11 to indicate months. 



4. A coinciding scale (B) extends along the 

 circumference of the larger disk but is gradu- 



ated by months, the last line of each being 

 prolonged centrally and the intervening spaces 

 labelled with the names of the corresponding 

 months. The index (&) of this scale is an 

 accentuated line representing December 31. 



5. An inner scale (C) on the same disk lies 

 around the circumference of the smaller disk 

 and, with (index c) on the same radius as 

 index h, divides the circumference into 100 

 equal parts. The lines are numbered by fives 

 from to 95 and represent successive years. 



6. Individual years are represented by scale 

 D on the smaller disk. This scale coincides 

 with scale G and is numbered from '05 to 

 1900 (equivalent to any even hundred year). 

 Attention is called to leap years by a dot on 

 every fourth line from that of '04 to that of 

 '98, inclusive. An erasable mark accentuates 

 the present year. 



Illustrations op the Use op the Cieculab Slide Rule 



(a) To determine the interval between two dates steps are taken is indicated by the numbers in the 



less than 365 days apart. (The order in which the schematic diagrams.) 



Scale A. I (3) At P, read the number of days (weeks or months) . 



Scale B. (1) Set P at later date. 1 (2) At index a, set earlier date. 



Example: Tind the interval between. May 21 and November 3. 

 Scale A. | 



(3) At P, read 166 days (23 weeks, 5 days, or 5 mos., 

 14 days) .' 



Scale B. I 



(1) Set P at November 3. 



In a leap year, if February 29 falls within the 

 interval determined, 1 is to be added to the num- 

 ber of days indicated at P. This result is best 

 obtained by advancing P one day in step (1), as 



Scale A. I 



(2) Atindexo, set May 21. 



follows : 



Example: Tor the interval from July 5, 1919, 

 to March 13, 1920. 



(3) At P, read 252 days (or 8 mos., 9 days) . 



(1) Set P at March 14 (March 13-1-1). 



(2) At index a, set July 5. 



(6) To find the age on September 17, 1920, of process were to be carried out) : 

 an individual born March 23, 1867 (if the entire 



1 Although the reading in total days is invariably 

 accurate (not considering leap year), readings in 

 months and days may deviate from the customary 

 calculation by not more than 3 days (because of 

 the inequality of the calendar months). After the 

 number of whole months is read, the number of 

 days in the remainder may be determined accu- 



Scale A. I (3) At P, read 5 months. 



rately by rotating scale B, carrying P (already set 

 at the later date) in the clockwise direction past 

 index a until the first day represented by the same 

 figure as the earlier date is at index a. Then, at 

 P on scale A, read the number of days in the re- 

 mainder. Thus, in the same example (from May 

 21 to November 3) : 



(5) At P, read 13 days. 



Scale B. 



(1) Set P at November 3. 



(2) At index a, set May 21. | (4) At index a, set October 21. 



