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 



Mean of twenty years 49'67. 



The mean temperatures of seven of the summers were 



1803 - 



1804 - 



1805 - 



1806 - 



- 67-3 



- 65-0 



- 62-2 



- 65-7 



1807 - 



1808 - 



1809 - 



Years. 

 1811 

 1812 

 1813 

 1814 

 1815 



Mean Temp. 

 51-6 

 47-8 

 48-6 

 48-2 

 50-0 



- 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 

 has traced a number of curves drawn over places which have the same, or nearly the 

 same, annual temperature, which he has called isothermal lines, 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 chart exhibits a 

 projection of the northern hemisphere, with the principal meridians and parallels of 

 latitude, and the isothermal lines described by Humboldt, 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. 



The isothermal line expressing 68 of temperature corresponds nearly to the northern 

 limit of the sugar-cane and coffee, and is generally the most southern limit of snow. In 



K K 2 



