Cause of the Glacial Period 
result in a difference of temperature ; and we shall endeavour to ascertain, with some degree of accuracy, 
the amount of variation as compared with that of the present day. 
It is known that the Gulf Stream receives a large accession of heat between the time that it enters 
the Caribbean Sea and leaves the Gulf of Mexico through the Straits of Florida. Off Cape San Roque 
the surface temperature is 73° Fahr., and on issuing from the Gulf it has risen to 86° Fahr., having in its 
passage gained thirteen degrees of heat. Increasing its latitude by ten degrees it loses but two degrees of heat, 
and with this temperature of 84° Fahr. it crosses the 4oth parallel, and spreads itself out over thousands 
of square leagues—carrying its warmth into the Arctic regions, and giving an increase of twelve degrees 
of temperature to the climate of the British Isles above that due to latitude.? 
Geographers have exhausted the powers of illustration in endeavouring to estimate the calorific 
effects of this great oceanic river. Croll states that each cubic foot of water carries from the tropics for 
distribution upwards of 1,158,000 foot-pounds of heat.2. The estimates of Maury and Herschell are 
still larger. According to the calculations of Meech the amount of heat transferred to the Arctic regions 
by the Gulf Stream is nearly half as much as that derived from the sun.? Lastly, Professor J. D. Forbes 
calculated that the quantity of heat thrown off in the Atlantic area by the Gulf Stream on a winter’s day 
would raise the temperature of the atmosphere which rests upon France and the British Isles from 
freezing-point to summer heat.‘ These statements will suffice to represent the effects of the Gulf Stream 
as it exists at the present day; we have now to inquire to what extent they would be modified under 
the view of the uprising ot a barrier of land connecting North and South America along the line of 
the Antilles. 
We have already seen. that the Gulf Stream gains thirteen degrees of heat between Cape San Roque 
and the Florida Straits. If we allow one degree for the increase between Cape San Roque and the entrance 
to the Caribbean Sea, the gain between this point and the Narrows will be twelve degrees. If, instead 
of entering the Caribbean Sea, the Stream passed northwards along the coast of continental land, it would 
have been deprived of twelve degrees of heat, but it would have gained some heat while flowing for 
1000 miles under the rays of a tropical sun. If we allow two degrees for this, then the total loss of heat 
on passing the coast of Florida will have been ten degrees as compared with that of the present day ; 
and instead of crossing the 4oth parallel with a surface temperature of 84° Fahr. as stated above, the 
Gulf Stream of the period referred to would have only had a temperature of. 74° Fahr., which would 
not be very much in excess of the summer temperature of the waters due to latitude at this parallel. 
We have now to inquire what would have been the effect of so great a reduction of temperature upon 
the climate of the North Atlantic and adjoining regions. A diminution of ten degrees of heat as compared 
with that of the present day would undoubtedly exercise a very important influence on the climate of the 
regions bordering the North Atlantic, and the coast and islands of the Arctic Ocean. Not only would the 
annual mean temperature be considerably reduced, but the increase of snow and ice over those tracts which 
are at present on the verge of perpetually glacial conditions would have the effect of lowering the 
temperature far beyond their own limits. As Lyell has truly observed, land in Arctic regions conduces 
to cold; and owing to the great extent of additional land in Europe and Asia which would have been 
brought under the influence of an arctic climate by the elevation of the land, the cold would be increased 
in the adjoining regions lying to the south. 
There is one way, perhaps the only way, by which we may indicate diagrammatically the climatic 
1 Croll calculates that on leaving the Gulf the mean temperature of the Stream is not under 65° Fahr. (C/imate ana Time, p. 25). 
2 Ibid. p. 25. 
3 Meech, Smithsonian Contributions to Knowledge, vol. ix. 
4 Forbes, Travels in Norway, p. 202. 
5 Rennell has calculated that the waters of the Gulf Stream on leaving the Gulf of Mexico with a surface temperature of 86° Fahr, are 
ten degrees above that of the Atlantic in the same latitude ; quot. by Lyell, Principles, 2nd edition, p. 244. A portion of the waters of the equatorial 
branch even now passes along the east coast of the West Indian Islands, ultimately joining the Gulf Stream. All this time, however, they are 
acquiring heat, but not to the extent which would be the case if they followed the main stream. ‘The amount, however, is unimportant in its 
bearing on the question before us, as the conditions of this branch of the equatorial current would have suffered no change. 
32 
