PHYSICAL CLIMATE. 



499 



different months, than a failure in the mean temperature, that disappoints the expecta 

 tions of the husbandman, and causes a scanty crop. A comparison of twenty years at 

 Geneva exhibits but slight variations of the mean temperature. 



Years. 

 1796 

 1797 

 1798 

 1799 

 1800 



Mean Temp. 

 49-3 

 50-5 

 50-0 

 48-7 

 50-5 



Years. 

 1801 

 1802 

 1803 

 1804 

 1805 



Mean Temp. 

 51-1 

 50-9 

 50-4 

 51-1. 

 47-8 



Years. 

 1806 

 1807 

 1808 

 1809 

 1810 



Mean Temp. 

 51-4 

 49-3 

 46-9 

 48-9 

 51-1 



Years. 

 1811 

 1812 

 1813 

 1814 

 1815 



Mean Temp. 

 51-6 

 47-8 

 48-6 

 48-2 

 50-0 



Mean of twenty years 49 '7. 



The mean temperatures of seven of the summers were 



1803 - 



1804 - 



1805 - 



1806 - 



- 67-3 



- 65-0 



- 62-2 



- 65-7 



1807 - 



1808 - 



1809 - 



- 68-2 



- 62-9 



- 63-0 



Mean of seven years 64-9 



M. Arago states, that in the two years 1815 and 1816, the last of which was destruc 

 tive to the crops in a great part of France, the difference of the mean annual temperature 

 was only two degrees from the standard. In the interval between 1803 and 1813, the 

 oscillations from the standard never went beyond it by more than 3 - 4, or fell short of it 

 by more than 2 '9. It thus appears, that, though temperature is continually varying, and 

 exhibits occasionally extraordinary degrees of heat and cold, there is great uniformity in 

 the mean condition of climate. It has been justly observed, that could we obtain a clear 

 insight into the complex machinery which regulates the seasons, we should behold the 

 same beautiful harmony, and the same system of compensation for temporary and appa 

 rent irregularities, which we are able to discern in the movements of the heavenly 

 bodies. 



In order to exhibit a more distinct view of the distribution of temperature, Humboldt 

 originated the convenient plan of connecting places which have the same, or nearly the 

 same, annual temperature, in lines on the charts, called isothermal, signifying lines of equal 

 heat. With indefatigable industry he made observations on both continents for this pur 

 pose, consulted the records of modern travel, and the European registers of observed 

 temperature; while the situation of particular plants furnished an index of practical 

 importance, and sometimes of greater certainty than the tables of thermometrical heat, on 

 account of the inaccuracy of instruments, or the unskilful use of them. The isothermal 

 lines strikingly illustrate the operation of other causes besides the solar action in the con 

 stitution of climate, otherwise they would be curves coincident with the parallels of lati 

 tude. In the torrid zone this is nearly the case ; but in the temperate and frigid zones 

 they become very irregular, ascending towards the pole in Western Europe and America, 

 and sinking towards the equator in Eastern Asia and America. The annexed meteorolo 

 gical map of the world, constructed by Dr. Petermann, shows the distribution of the tem 

 perature of the air by isothermal lines, as far as adequate data exist for the purpose, the 

 entire course of which remains to be ascertained. The figures attached to each curve 

 indicate the mean annual temperature of the region traversed by it. A glance at the 

 map shows the divergence of the lines of equal heat from those of equal latitude. In 

 Europe they are convex with reference to the Pole ; in Asia and America, concave ; 

 becoming less and less concave as they approach the equator. 



In the northern hemisphere, the curve line on the map, indicating the mean annual 

 temperature of 80, crosses central America about the Gulf of Honduras, passes north of 



