352 



many disturbing influences from heat and moisture so difficult to 

 escape on board ship. In this respect additional precautions must 

 be used if night observations are to be required, since the ordinary 

 difficulties are necessarily much enhanced by the employment of arti- 

 ficial light. Amongst the instructions which will be required, perhaps 

 there will be none which will need to be more carefully drawn, than 

 those for obtaining the correct temperature of the external air under 

 the continually varying circumstances that present themselves on 

 board ship. 



In regard to land stations, Professor Dove's tables have shown that 

 data are still pressingly required from the British North American 

 possessions intermediate between the stations of the Arctic Expeditions 

 and those of the United States ; and that the deficiency extends 

 across the whole North American Continent in those latitudes from 

 the Atlantic to the Pacific. Professor Dove has also indicated as 

 desiderata, observations at the British Military stations in the 

 Mediterranean (Gibraltar, Malta and Corfu), and around the Coasts 

 of Australia and New Zealand : also that hourly observations, 

 continued for at least one year, are particularly required at some 

 one station in the West Indies, to supply the diurnal corrections for 

 existing observations. 



Whilst the study of the distribution of heat at the surface of the 

 globe has thus been making progress, in respect to the mean annual 

 temperature in different places, and to its periodical variations in 

 different parts of the year at the same place, the attention of phy- 

 sical geographers has recently been directed (and with great promise 

 of important results to the material interests of men as well as to 

 general science), to the causes of those fluctuations in the tempera- 

 ture, or departures from its mean or normal state at the same place 

 and at the same period of the year, which have received the name 

 of " non-periodic variations." It is known that these frequently 

 affect extensive portions of the globe at the same time ; and are 

 generally, if not always, accompanied by a fluctuation of an opposite 

 character, prevailing at the same time in some adjoining but distant 

 region ; so that by the comparison of synchronous observations a 

 progression is traceable, from a locality of maximum increased heat 

 in one region, to one of maximum diminished heat in another 

 region. For the elucidation of the non-periodic variations even 



