Eruption of Mount Etna, in 1819. 



[To the Editor of the Quarterly Journal of Science, &c.] 



Sir, 



I TAKE the liberty to request that you will allow me a small 

 space in your Journal^ to point out what I conceive to be 

 an error, which has somehow crept into several of our best ele- 

 mentary works on Mechanics. 



The error I allude to is in the theory of the wedge, Mr. Ni- 

 cholson's Introduction to Natural Philosophy is the only one of 

 the few works, to which I liave access, but what appears to me 

 to make the advantage gained by this instrument twice as great 

 as it ought to be considered. 



It is probable that this error (if it be such) found its way into 

 Dr. Olinthus Gregory's otherwise very excellent Treatise, from 

 the theory having but little engaged his attention, being, as he 

 observes, of no very great practical utility. The proposition con" 



