On the Derivation of the Word Theodolite. 287 



the difference of course being greatest in the daytime; thus 

 then the daytime is more favourable for the descent of air 

 from the upper strata than in winter. 



Hence then I think it may be concluded that the air of the 

 upper current becomes relatively colder than the lower strata 

 of the atmosphere by loss of its heat by its own radiation, and 

 that when the cold has arrived at a certain degree, it de- 

 scends, if other conditions which influence its descent are fa- 

 vourable; these conditions being the state of the temperature 

 and pressure of the air of adjacent latitudes, by which the 

 force urging forward the surface current of the atmosphere is 

 affected; and the state of the opposite currents with respect 

 to aqueous vapour. 





XLVI. On the Derivation of the Word Theodolite. 

 By Professor De Morgan *. 



T^HE word theodolite has puzzled all who have tried to trace 

 p it to its origin. Some have connected it with the roots 

 of dedofiat, and SoXivo9, and made it a seer of lengths, though 

 the instrument neither does, nor ever did, see anything but 

 angles. In a modern dictionary of good reputation, it is con- 

 nected with deaofxai and 80X09, and made a seer of stratagems, 

 which might apply to a telescope: but unfortunately the use 

 of the term theodolite was prior to the invention of the tele- 

 scope. 



The word is exclusively English, never having obtained any 

 mention from foreigners till comparatively recent times. The 

 Encyclopedic Melhodique (1789) does indeed give the word 

 without allusion to its origin; but Saverien's dictionary (1753) 

 says that the theodotile (as it is spelt) is an instrument used 

 by the English, much resembling the graphomelre. 



I find that the use of the word runs back to the " Geometri- 

 call practise named Pantometria," begun by Leonard Digges, 

 and finished by Thomas Digges his son (published London 

 1571, quarto, reprinted in 1591). But it seems as if the name 

 was not then new. Chapter 27 is on " the composition of the 

 instrument called Theodelitus," and it is plain from various 

 modes of speaking that the word is here an adjective or par- 

 ticiple. This "circle called Theodelitus," or "planisphere 

 called Theodelitus," is nothing but a graduated circle with a 

 revolving diameter furnished with sights, and placed horizon- 

 tally. Held vertically, it would have been the astrolabe of 

 the period, and nothing else. In Leybourn's ' Compleat Sur- 

 veyor,' 1657, we learn that the altitude circle was sometimes 

 * Communicated by the Author. 



