lx REPORT—1857. 
physical changes of our globe. According to the computations of Bessel, 
25,000 cubic miles of water flow, in every six hours, from one quarter of the 
earth to another. The store of mechanical force is thus diminished, and the 
temperature of our globe augmented, by every tide. We do not possess the 
data which would enable us to calculate the magnitude of these effects. 
All that we know with certainty is, that the resultant effect of all the thermal 
agencies to which the Earth is exposed, has undergone no perceptible change 
within the historic period. We owe this fine deduction to Arago. In order 
that the date palm should ripen its fruit, the mean temperature of the place 
must exceed 70° Fahr. ; and, on the other hand, the vine cannot be cultivated 
successfully when the temperature is 72° or upwards. Hence the mean 
temperature of any place, at which these two plants flourished and bore fruit, 
must lie between these narrow limits, 7. e. could not differ from 71° Fahr. by 
more than a single degree. Now, from the Bible we learn that both plants 
were simultaneously cultivated in the central valleys of Palestine, in the time 
of Moses, and its then temperature is thus definitively determined. It is the 
same at the present time; so that the mean temperature of this portion of 
the globe has not sensibly altered in the course of thirty-three centuries. 
The future of physical science seems to lie in the path upon which three 
of our ablest British physicists have so boldly entered, and in which they have 
already made such large advances. I may therefore be permitted briefly to 
touch upon the successive steps in this lofty generalization, and to indicate 
the goal to which they tend. 
It has been long known that many of the forces of nature are related. 
Thus heat is produced by mechanical action, when that is applied in bringing 
the atoms of bodies nearer by compression, or when it is expended in friction. 
Heat is developed by electricity, when the free passage of the latter is impeded ; 
it is produced whenever light is absorbed; and it is generated by chemical 
action. A like interchangeability probably exists among all the other forces 
of nature, although in many the relations have not been so long perceived. 
Thus the development of electricity from chemical action dates from the 
observations of Galvani; and the production of magnetism by electricity 
from the discovery of Oersted. 
The next great step was to perceive that the relation of the physical forces 
was mutual; and that of any two, compared together, either may stand to 
the other in the relation of cause. 
With respect to heat and mechanical force, this has been long known. 
When a body is compressed by mechanical force, it gives out heat; and, on the 
‘other hand, when it is heated, it dilates, and evolves power. The knowledge 
of the action of electricity, in dissolving the bonds of chemical union, followed 
closely upon that of the inverse phenomenon, and the discovery of electro- 
magnetism by Oersted was soon followed by that of magneto-electrictty by 
Faraday. With reason, therefore, it occurred to many minds that the relations 
of any two of the forces of nature were mutual ;—that that which is the cause, 
