20 Prof. Snell on new articles of Philosophical Apparatus. 
surface of the earth is destitute of soil, and is formed of bare 
and almost naked rocks that show few traces of vegetation. 
Although the quotations from travellers lack that accurate ex- 
amination that is necessary to a determination whether the sur- 
faces thus described have been exposed to the action of violent 
and long continued currents, yet they have their weight, when 
considered in connection with the effects of known physical 
causes, and render it more than probable that the currents under 
consideration have flowed from the polar regions towards the 
equator, and from the tropics towards the poles, when this con- 
tinent was beneath the ocean, and that the matter’of the vast 
deposit of the sedimentary rocks of the United States was 
washed away by these great equilibrating currents from the bed 
of the ocean, from reefs, islands and coasts, and finally deposited 
from suspension over the great area where we now find it exposed 
to observation. 
(To be continued.) 
Arr. Il.—Account of some new Articles of Philosophical Appa- 
ratus ; by Prof. E. S. Snexx, of Amherst College. 
1. Instruments for illustrating sea waves, and waves of sound. 
Tue idea of constructing apparatus for such purposes was first 
suggested to me by seeing acut of Prof. Powell’s machine for 
exhibiting plain, circular and elliptical polarization of light. ‘The 
thought struck me that every species of wave motion might 
be produced mechanically, and that such visible representations 
might be advantageouly used in giving instruction. I formed 
the design, therefore, of providing myself with a series of instru-— 
ments to illustrate the oscillatory* waves of the sea, the acoustic 
waves of the air, and the undulations of the luminiferous ether, 
both ordinary and polarized. For the two first, which presented 
the least difficulties, I soon devised and executed a simple and 
convenient mechanism; and the instruments have more than 
answered my expectations in their operation and in their value as 
means of instruction. 
* See Russell’s classification of waves, Reports of British Association, 1837, p. 
425, and Vol, xxxvuii, p. 100, of this Journal. 
