Mr. J. Cockle on the Derivation of the word Theodolite. 293 



referred to, applies the term alidade to a revolving diameter. 

 At least, such seems to be the effect of his statement, that " A 

 ruler with sights, travelling upon a graduated circle, was a 

 constituent part of various astronomical instruments imported 

 into Europe from the East, and was accompanied by the 

 Arabic term alhidada to express it:" combined with a remark 

 in the paragraph preceding that statement. But it will be 

 observed that it is only by implication (if, indeed, at all) that 

 Mr. De Morgan excludes the application of the term alidade 

 to a revolving i^adius, and, consequently, without contradicting 

 his high authority, I may express a doubt whether any more 

 is essential to the definition of an alidade than that it is a ruler 

 with sights travelling on a circle or portion of a circle, and 

 that it is as applicable to a revolving radius as to a revolving 

 diameter. It is true that, in chapter 29 of Longimetra, the 

 term " index with sightes " is used with reference to the 

 "square Geometricall," and "Alhidada" with reference to 

 " Theodelitus ;" but then the words " er* index with sightes" 

 immediately follow the word "alhidada," and would rather 

 seem to show that the expressions are synonymous. Why, 

 then, are they apparently distinguished in the manner just 

 mentioned ? It may be said, because the alhidada is a double 

 index, whose diversity from the single one is manifested by 

 the first plate to chap. 29, where the alidade occupies the 

 right, and the index the left-hand side. But on the other side 

 it may be urged, and perhaps not without effect, that, although 

 the word " Theodelitus " occurs before that chapter, yet the 

 term "alhidada" does not\ and further, that the more com- 

 plicated nature of the "instrument Topographicall" described 

 in chap. 29 renders the use of the word alhidada necessary, 

 in order to distinguish the index of the theodolite from that 

 of the square, and that it is only for convenience and distinct- 

 ness that it is there used. 



The view contained in the preceding paragraph appears to 

 be confirmed by a consideration arising upon the first of the 

 points above alluded to. It will be found that the chapter 27 

 of Longimetra (" The composition of the instrument called 

 Theodelitus") contains no mention whatever of the term ali- 

 dade. We are told that "The index of that instrument with 

 the sightes, &c. are not unlike to that whiche the square 

 hath : . ." No peculiarity of its index is adverted to either in 

 that or in the succeeding (28th) chapter, in which the "index " 

 is mentioned no less than three times. Add to this, that the 

 theodelitus may {Long, chap. 27 and 28) be semicircular, in 

 which case a single index would be used, and the alidade 

 (even if it meant a double index) would cease to be identified 

 • Sic in original. Probably a misprint for or? 



