PHYSICAL CLIMATE. 



485 



1. The geographical position of a country with reference to the equator is one of the 

 leading circumstances by which its temperature is determined. At the equator, and 

 within the tropics, the greatest heat is experienced, because the sun is always vertical to 

 some place within those limits, and the solar action is the more intense in proportion as 

 the rays are perpendicular to the earth. As we recede from the equator, north or south, 

 their direction becomes more oblique, and less influential in promoting temperature. It 

 is for this reason that a declivity towards the equator, which receives the solar rays more 

 directly than a level surface, is always the warmest, and hence the importance attached to an 

 inclination of the surface with a southern aspect in our climate. The latitude of a place is 

 therefore a prime determining cause of the temperature to which it is subject, a decrease of 

 heat taking place with an increase of distance from the equatorial localities, though with 

 various modifications, which will be hereafter noticed. One exception taken to this general 

 rule may here be stated. It is true of countries lying between the tropics and the poles, 

 that heat decreases with an increase of latitude, but it is thought not to be true of coun 

 tries between the tropics and the equator. Tracing the path of the sun as delineated upon 

 an artificial globe, we perceive an advance of 12 of latitude made in the first month after 

 the equinox, only 8 in the second, and but 3^ in the third ; and upon retiring from the 

 tropic to the equator, he follows the same course inversely, traversing 3^ of latitude in 

 the first month, 8 in the second, and 12 in the third. It follows, therefore, that to all 

 places situated within 3^ of each tropic, his rays at noon are vertical, or make but a 

 slight angle for two months respectively ; whereas those places which are under the 

 equator have but the sun as near their zenith for about a week at each equinox. From 

 this circumstance it has been inferred that in receding from the equator there is no 

 decrement in the mean annual temperature till we have passed 23^, the latitude of the 

 tropics. This cannot yet be regarded as an established fact, though it is quite certain 

 that the mean temperature in summer near the tropics is higher than near the equator. 

 In the northern hemisphere the countries where the greatest heat is experienced the 

 banks of the Senegal, the Tehama of Arabia, andMekran in Beloochistan coincide with 

 the tropic of Cancer ; and it has been observed that the snow-line of the Andes in 17 



Plains of Bclooclrlstnn. 



