408 Prof. De Morgan on Fernel's Measure of a Degree. 



drurets of arsenic and antimony, 1 have invariably found, 

 unless they be excessively minute and attenuated, that each 

 manifests its characteristic colour, the brown arsenical stain 

 appearing around the dark and almost black anlimonial stain : 

 this position of the arsenical stain is due, no doubt, to its 

 greater volatility, for in collecting pure arsenical stains, the 

 central portions are almost always wanting when the porcelain 

 is held in such a manner as to be perpendicular to the axis of 

 the flame : this is not the case with the antimonial stain when 

 collected under precisely similar circumstances. 

 Liverpool, Feb. 14, 1842. 



LXI. On Fernel's Measure of a Degree. By Professor 

 De Morgan. 



To the Editors of the Philosophical Magazine and Jour?ial. 



Gentlemen, 



T MUST trouble you with one more communication on the 

 -*- subject of Fernel's degree, as I have now direct evidence 

 that all I have heretofore advanced is perfectly correct. 



I will first recapitulate the steps of the discussion. In the 

 Magazine for December, I called your attention to the fact 

 that the French historical writers had grossly misinterpreted 

 Fernel's account of his own measure. In that of February, I 

 made it appear from his own words that if those historians 

 were correct, Fernel must have been in the habit of taking 

 steps of 38 inches each, at least. Also in February, my friend 

 Mr. Galloway replied to my first communication, maintaining 

 from the probability of the case, and from the authority of 

 the most celebrated astronomers and metrologists of the seven- 

 teenth and eighteenth centuries, that the French historians 

 were substantially right, and that FerneVs geometrical foot was 

 the French foot of his time, which (we both agreed) differed by 

 no material length from its present value. In the Number for 

 March, I replied to Mr. Galloway, showing, as I thought, that 

 throughout the sixteenth century there was a system of mea- 

 sures current among mathematicians, expressly intended to 

 get rid of the great diversities of common measures, by intro- 

 ducing the smaller diversities of measures directly derived 

 from the human body: in confirmation of this, I produced the 

 direct authority of Clavius, who lived through a great part of 

 the century in question. 



In looking i'urther through writers of the sixteenth century 

 at the British Museum, I fell upon the Mo7ialosj)hccrium of 



