108 British Association for the Advancement of Science. 
steel to the tube, at the point at which the expansion of the tube 
is the same as that of the rod; so that, as the steel rod expands 
_ _- downwards, and is lengthened by heat, the zinc tube expands up- 
wards in the same degree; and therefore, if the lengths of the | 
rod and the tube be rightly proportioned, the pendulum may be 
regarded as of invariable length. Some other additional appara- — 
’ tus is devised in order to the greater perfection of this pendulum, 
‘but the description is too long for this place. 
. The Secretary read the report of the Committee, consisting ary 
. = 06 Sir J. Herschel, Mr. Whewell, Mr. Peacock, and Prof. Lloyd, © _ 
2, ~ appointed to represent to government the resolutions adopted by 
£- - the Association in August, 1838, recommending that Magnetic Ob-. 
. servatories be established in various parts of the British dominions, ” 
and that a naval expedition be fitted out for the purpose of de- 
termining, | by observations, the magnetic direction and intensity, 
1 high n latitudes, between the meridians of New Hol- 
pen ‘agid.end: Cape Horn ez, Itis well known that this application was 
eminently successful ; that the Antarctic Expedition sailed in 
ea the summer of 1839, end that efficient measures have been ta- 
; < ken to secure a magnificent system of magnetic observations. 
Prof. Powell made a communication on certain points in the 
~~ wave-theory as connected with Elliptic polarization. Its object 
~ -was to set forth a general statement of some material conditions 
. which involve in a common relation the theory of dispersion, of 
the wave surface and of elliptic polarization. | 
.  . Mr. E. Hodgkinson gave an account of experiments made by | 
_ order of the Association on the temperature of the Earth in the | 
| 
ad 
: 
- ae 
-. deep mines of Lancashire and Cheshire. Satisfactory results 
~. appear not to have been yet ard attained, and the pee“ 
are to be continued. | “= 
- Prof. Forbes submitted a sia Gf Shaarvations saa by order = 
of the Association on the temperature of the Earth at different | 
depths near Edinburgh. These observations were commenced 
in February, 1837, and have been regularly continued since- 
_ The object was to ascertain the conducting power for heat, of . 
different soils, and the measure of the sun’s influence at different | 
hs under similar external circumstances. At each station 
were sunk to the depths of 3, 6, 12, and 24 | 
t és ectively, the tubes of each being coniell we * 
= a pats eee exposed side by side. The 
ie * 
- = 253 ee a ca 
pie = 6 eee 
% = € - ‘ 
a wt ihe - * 
ee. a re ~— 
g chide ae : a“ 3 sid * 
= Rey 2S Pee 2 i aoe Pe 
