PREFACE. vii 
and whose inventive genius has led to many im- 
provements on them. I need only remark, that he 
has made nearly the whole instruments which were 
used by our late highly distinguished Professor of 
Natural Philosophy, Sir John Leslie, as also those 
used by Sir David Brewster, in his experiments on 
Natural Philosophy, and more particularly such as 
were connected with his admirable discoveries con- 
cerning the polarization of light. 
When Travellers and Naturalists go into foreign 
lands, or even traverse the more remote districts of 
Britain, or the continent of Europe, they should not 
confine their researches to the collecting of natural 
objects alone ; their attention should be extended to 
geographical and meteorological observations, the 
altitude of mountains, lakes, and sources of rivers, 
above the level of the sea; the moisture and parti- 
cular state of the atmosphere in those situations ; 
the temperature of hot springs; their relation to 
volcanic causes: for all which purposes instruments 
are made, by which these things can be easily de- 
termined. 
On these interesting points, n much yet remains to 
be done even in Great Britain. : 
Tak: 
