Natural Phihsopliy — Optics and Acoustics. S03 
the mountains, but which did not exist at ^ lower level, so as to 
affect the inferior parts of the mountains. 
is. Dr Watfs Theory of the Rainbow. — We have no 
doubt that our readers will partake in the surprise which 
we ourselves experienced, at seeing it gravely maintained 
that the Rainbow is not produced from ram. The learned 
Dr Watt of Glasgow has maintained this hypothesis in the 
Annals (f Philosophy for February 1819, p. 131 , and has gone 
so far as to say that he considers his “ hypothesis as in a 
great measure established,” He supposes that the rainbow is 
nothing more than a spectrum produced by the refraction of 
the edge of a cloud, and that the rainbow must always dis- 
. appear when the sun emerges from behind this magical prism. 
The following are a few out of many reasons why such a 
mode of formation is absolutely impossible i 
1. A cloud with two perfect surfaces, capable of producing 
such a distinct spectrum, is a thing quite inconceivable. 
2. In Order that the spectrum may be always concave down- 
wards, like the rainbow, the cloud must always take care and 
place its refracting angle mathematically in one position. 
3. In order that the bow may appear both on the right and 
left of the observer, as it does in nature, the prismatic cloud 
must have the common section of its two refracting planes, of a 
circular form. 
We cannot allow ourselves to offer any defence of a theory so 
palpably true as the ordinary theory of the rainbow. If any 
doubt were attached to it, it must have been completely removed 
by the discovery made by Dr Brewster, ( Treatise on Philosophi-' 
cal Instruments, p. 350), that the light of the rainbow is actuak 
ly polarised light, in consequence of its having suffered reflection 
nearly at the polarising angle from the posterior surface of the 
drops of water. Such a change upon the light could not possi- 
bly have been effected by passing through any prism whatever. 
This indeed is an experimenlum crucis, which demonstrates 
Newton’s theory to be correct, and Dr Watt’s erroneous. 
ACOUSTICS. 
13. Velocity of Sound. — A series of experiments on the veloci- 
ty of sound has lately been performed at San Jago in Chili, by 
M. D. Josef de Epinosa, and D, Felipe Eauza, who obtained 
the following results ; 
