200 Proceedings of the Eoyal Society 
divisor or multiplier, could be kept out as the maximum unit. 
This would introduce the decimal principle into the coinage, with- 
out sacrificing either the pound or the penny, which it has hitherto 
been declared impossible to do, while it would completely revolu- 
tionise our arithmetic, and bring our money into accord with the 
whole world. By this slight alteration, making no disturbance in 
the names or values of the coinage — affecting nobody injuriously, 
but everybody beneficially — a change of notation from the present 
£ s. d. is made imperative upon no one who prefers it ; but the 
great mass of traders may at once keep their accounts in pounds 
and 1200ths, while the larger merchants and bankers may keep 
them in pounds and 120ths, greatly to the diminution of figuring 
and arithmetical labour, while the conversion of the decimal into 
s. and d. is the simplest mental operation. 
4. On the Elevation of the Earth's Surface Temperature 
produced by Underground Heat. By Professor W. 
Thomson, F.K.S., E.E.S.E. 
Peclet found, by his own experiments, that a body with any 
common unpolished non-metallic surface, kept by heat from within 
at 1° higher temperature than that of the air and other objects round 
it, loses heat from each square metre of surface at the rate of about 
nine kilogramme-water thermal units per hour, or, which is the 
same, i-oVv^h of a gramme-water unit from each square centimetre 
per second. The mean conductivity of the three Edinburgh strata, 
in which Principal Forbes’s underground thermometers were 
placed, is 2| grain-water units per second per square foot per 1° 
per foot rate of variation of temperature, as I have shown pre- 
viously.* 
That of the G-reenwich stratum is 2‘6, in terms of the same units, 
according to Professor Everett’s recent reductions ; and that of 
certain strata (clay and sand) in Sweden is 1’64, according to 
o 
Angstrom (Poggendorff’s Annalen^ last volume of 1861). The 
mean of these three numbers is 2*33, which, reduced to the unit 
* Trans. K.S.E., April 1860, “ On the Eeduction of Observations of Under- 
ground Temperature,” § 42. 
