a Large Refracting Telescope. 329 



to justify farther attempts, I have no doubt the Board will, as 

 far as is consistent with its constitution, forward the prosecution 

 of the experiments, and ultimately the construction of an achro- 

 matic telescope, which shall exceed in aperture and power any 

 instruments of the kind hitherto attempted. It is hardly neces- 

 sary for me to add, that in such case it will give me great satis- 

 faction to undertake the direction of these operations, with the 

 aid of Messrs W. and T. Gilbert, to whose liberal and scientific 

 views, as well as to their practical skill and ingenuity, I have 

 been much indebted for having thus far proved the practicabi- 

 lity of my proposition. 



Peter Barlow. 



Royal Military Academy 

 I3th October 1827- 



•} 



On the Principal Causes of the Difference of Temperature on 

 the Globe. By Baron Alexander Von Humboldt. * 



L HE distribution of heat over the globe, has for many years 

 formed one of the principal objects of my researches. This sub- 

 ject is intimately connected with the local differences of the pro- 

 ductions of nature, with the agriculture and the commercial in- 

 tercourse of nations, and even, in several respects, with their mo- 

 ral and political situation. The time is past when we were satis- 

 fied with some undefined views on the difference of geographical 

 and physical climates, and when all the modifications of tempera- 

 ture were ascribed either to the shelter afforded by ridges of 

 mountains, or to the various elevations of the surface of the 

 earth. We have seen, that the remarkable differences of 

 climates which we perceive in large tracts of country, under the 

 same latitude, and on the same level above the surface of the 

 sea, do not arise from the trifling influence of individual locali- 

 ties, but that they are subject to general laws, determined by the 

 form of the continents in general, by their outlines, by the state 

 of their surface, but particularly by their respective positions, 

 and the proportion of their size, to the neighbouring seas. The 



* Extract from a public Lecture delivered in the Koyal Academy at Berlin, 

 on the 3d of July 182?, 



2 



