XI] 



OF THE NAUTILUS SHELL 



519 



line was drawn across its whorls, and their widths were measured 

 upon it in succession, beginning with the last but one. The 

 measurements were, as before, made with a fine pair of compasses 

 and a diagonal scale. The sight was assisted by a magnifying 

 glass. In a parallel column to the following admeasurements 

 are the terms of a geometric progression, whose j&rst term is the 

 width of the widest whorl measured, and whose common ratio is 

 M804. 



Fig. 262. 



Widths of successive 



whorls measured in inches 



and parts of an inch 



1-31 

 112 



•94 



•80 



•67 



•57 



•48 



•41 



Terms of a geometrical progression, 



whose first term is the width of 



the widest whorl, and whose 



common ratio is 1-1804 



1-31 

 1-1098 



•94018 



•79651 



•67476 



•57164 



•48427 



•41026 



The close coincidence between the observed and the calculated 

 figures is very remarkable, and is amply sufficient to justify the 

 conclusion that we are here dealing with a true logarithmic 

 spiral. 



Nevertheless, in order to verify his conclusion still further, 

 and to get partially rid of the inaccuracies due to successive small 



