Mr. Earnshaw's Ansxver to Prof. Moseley. 271 



its irregular vascular inner surface as the locus of accurate 

 convergency. 



To persons who are timid, or have weak eyes, it may be 

 interesting to know that these experiments succeed nearly as 

 well on the closed as on the open eye, as far as concerns the 

 distinct exhibition of the vascular spectrum, the difference 

 being scarcely more than may be ascribed to the light which 

 then reaches the cornea by diffusion through the lid. But in 

 fact, if the air is mild, the mere examination of the vessels 

 (Arts. 3, 4, 5.) is less trying to the eye, than continued reading, 

 or exposure to a strong light. W C H 



XLVII. On Prof. Moseley's Explanation of the " Principle 

 of Least Pressure" inserted in the Lotid. and Edinb. Phil. 

 Mag. for March. By S. Earnshaw, B.A., Fellow of the 

 Cambridge Philosophical Society. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 

 T AM fully aware of the great importance of the question 

 *- of statical resistances, which Professor Moseley has under- 

 taken. The subject is one of peculiar difficulty, and has de- 

 fied the attempts of some of the ablest mathematicians to con- 

 quer it; it is therefore the more necessary that every theory 

 which is now proposed should be scrutinized with very great 

 care before it be received and acted upon. These considera- 

 tions will screen me, I hope, from a charge of obstinacy when 

 I inform Mr. Moseley that his explanation has had the effect 

 of strengthening rather than of removing my previous objec- 

 tions to his theory. I shall therefore now set before him, at 

 some length, my reasons for differing from him on this sub- 

 ject. 



In the first place, Mr. Moseley denies " that if the foices of 

 the system C are not parallel, their actions must of necessity 

 propagate pressures." Now, for the sake of avoiding the 

 complexity of reasoning in general terms, let us suppose the 

 body acted upon by gravity only, and substitute the system C 

 for the reactions B. Then the system A balances the system 

 C. But the former = weight of the body, and acts in a ver- 

 tical direction, and cannot, therefore, propagate horizontal 

 pressures ; for the resolved part of a vertical force is equal to 

 zero in a horizontal direction. Also the forces C (being ob- 

 lique to each other by hypothesis,) may be resolved into a ver- 

 tical and a horizontal set; the forces of the former set exactly 



