610 
The mean of both series may be tolerably represented by a uni- 
formly diminishing conductivity as the temperature increases. When 
reduced* to the usual units of conducting power expressed in terms 
of the amount of heat necessary to raise by 1° Centigrade a cubic 
foot or a cubic centimetre (one gramme) of water respectively, we 
have the following absolute measures : — 
Conducting Power of Wrought Iron. 
Temperature, Units, the Foot, Minute, Units, the Centimetre, 
Centigrade. and Cent. Degree. Minute and Cent. Degree. 
0° -0133 12-36 
50 *0120 ...... 11*15 
100 -0107 9'94 
150 '0091 8'73 
200 -0082 7'62 
It is to be observed that thermometric readings have not yet 
been finally corrected, so that these numbers may receive some 
slight modification. The author hopes to complete the verification 
of the calculations, so far as wrought iron is concerned, in the course 
of the present summer. The state of his health has been the cause, 
not only of the suspension of the experiments, but of the long delay 
which has taken place in publishing the results so far as obtained. 
2. On Certain Vegetable Formations in Calcareous Spar. 
Bj Principal Sir David Brewster. 
3. On the Existence of Acari between the Laminse of Mica 
in Optical Contact. By Principal Sir David Brewster. 
4. On the Secular Cooling of the Earth. By Professor 
William Thomson. 
The fact that the temperature of the earth increases with the 
depth below the surface, implies a continual loss of heat from the 
interior by conduction outwards, through op into the upper crust. 
Since the upper crust does not become hotter from year to year, 
there must therefore be a secular loss of heat from the whole earth. 
It is possible that no cooling may result from this loss of heat, but 
* The numbers in the preceding table refer to the thermal capacity of iron 
instead of water. 
