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SECOND REPORT - 1 832 . 
their apartments ; and more lately I have had constructed for my 
own use, a portable barometer intended for a projected tour on 
the Continent, in which I have endeavoured to unite the great 
requisites of accuracy, portability, and security from accident. 
These properties have been considered so much opposed, that 
hardly any of the various inventions, or modifications of inven¬ 
tions, which are constantly brought before the scientific world, 
can prefer a claim to all three. Perhaps the most portable baro¬ 
meter susceptible of any considerable accuracy which is in use, 
is that of M. Bunten, a modification of Gay-Lussac’s, having a 
safety cavity in glass half-way up the tube for stopping the 
progress of any air which may pass the syphon at the bottom*. 
The instrument is well mounted and graduated ; but the prin¬ 
cipal defect, as well as in Gay-Lussac’s, arises from the friction 
of the mercury in the shorter leg of the syphon, where it never 
fails to oxidize,—and from the contraction of the scale, which is 
necessarily much shortened *f\ I am informed however, by my 
friend Captain King, that he found it on the whole a satisfac¬ 
tory instrument and extremely portable J. 
An important source of error in portable barometers is the 
difficulty of finding the actual temperature of the mercury; with 
a syphon barometer of the kind just mentioned, Signor Bellami 
and M. Legrand have pointed out methods of converting the 
instrument into a temporary thermometer, and thus showing its 
own temperature §. 
Among other ingenious devices for diminishing the risk of 
breakage, one has been proposed by Mr. Jones of Charing 
Cross, by constructing the tube wholly of iron; such an one 
has, I believe, been actually completed, but I have not heard of 
its success. Mr. Robinson, of Devonshire Street Portland Place, 
London, has lately constructed a barometer in which the tube 
consists of two parts, capable of being screwed together at the 
moment of observation. 
The adjustment of the lower level of the barometric column 
is one of the most difficult parts of the apparatus. I am con¬ 
vinced that the French method of bringing up the mercury in a 
transparent cistern till a fine fixed point impinges on its sur¬ 
face, will gradually come more into use in this country, when¬ 
ever really good barometers become an object of attention, 
which at present can hardly be said to be the case. A most beau- 
* A good account of this barometer, with M. Arago’s Report upon it, made 
to the Academy of Sciences, will be found in Ferussac’s Bulletin des Sciences 
Mathematiques , x. 187. f In fact it is generally reduced to one half. 
X Since writing the above I have used a barometer upon Gay-Lussac’s con¬ 
struction with great satisfaction among the Alps. Dec. 1832. 
§ Bulletin des Sciences Mathematiques, tom. x. 
