58 NOLLOTH. HOPKINS. [April 14, 1856. 



4. Letter from Commander Nolloth to Commodore Trotter, with 



Enclosures. 



Communicated by the Admiralty. 



" . . . . The Governor assured me that he had given ample 

 directions, providing for Dr. Livingston's welfare, should he make his 

 appearance at Tet6 ; and that he had ordered his own dwellings at that 

 place, and at Senna, to be placed at his disposal ; and he was good 

 enough to say that he would send still further directions." 



Commander Nolloth encloses a detailed statement of the time occu- 

 pied in voyages and Journeys between Quiliniane and Tete at different 

 times of the year. The most favourable voyage down river occupies 

 three days, and the most unfavourable voyage up river forty days. 



He also sends a translation of the Mozambique Government Gazette 

 of March 17th, 1855, containing a few of the names of the places through 

 which the Moors passed, who journeyed from Zanzibar to Benguela and 

 back again to the East Coast by a different route. They left Benguela 

 June 7th, 1853, and arrived at Mozambique November 12th, 1854.* 



5. On the Causes of Dryness in certain Arid Districts. By 

 Thomas Hopkins, Esq., of Manchester. 



Mr. Hopkins examines into the nature and causes of those winds which 

 blow in regular directions. He argues against the principle of the 

 Hadleian theory, and disputes many facts commonly advanced to sustain 

 it. He denies any general and obvious movement of the atmosphere 

 from the polar to the tropical regions, and asserts, on the other hand, 

 that all regular winds blow to one or other of the following places — they 

 being the great mountainous districts of the globe, which arrest vapour 

 and produce heavy rains, and which he calls " areas of condensation :" — 

 the Andes, the Himalaya, the lofty islands of the E. Archipelago, and 

 the snow mountains of Tropical Africa. Other mountains produce 

 similar effects, but of secondary importance. 



Mr. Hopkins' hypothesis is that mountains condense vapour from 

 the air that surrounds them — that this process of condensation liberates 

 heat which raises the temperature of the air — that a partial vacuum 

 results from this condensation and heating, which is filled by indraughts 

 of air from all sides. These create ascending vortices, and cause a 

 boiling up and an overflowing of large masses of the atmosphere in the 

 higher regions ; here they diffuse themselves, and in time descend, per- 

 haps in far distant places, or they may press upon and put in motion the 

 air that lies beneath them. As the wind passes from colder latitudes 



♦ For ftirther particulars about their journey, See p. 75. — Ed. 



