REPORT ON ATMOSPHERIC CIRCULATION. 3 



ground as ascertained by objects on sbore. These several results, independent of the 

 ship reckoning, are given in the account of the voyage. It has been disputed whether 

 the difference in a day's run between the position of the ship by dead reckoning and 

 by astronomical observation can be regarded as giving with sufficient accuracy the 

 movement of the surface water. But the long experience of navigators proves that 

 with due care this result furnishes a fair approximation to the truth, and especially so 

 when the astronomical observations are made at frequent intervals during the twenty- 

 four hours. Much care was taken during the voyage, by due attention to the steerage 

 and estimation of the leeway, to keep an accurate reckoning, and also to ascertain 

 frequently the latitude and longitude by astronomical observation. The surface set 

 deduced by these means was found in the majority of cases to be continuous during 

 the day, and to agree in a striking manner with the surface current, as found by 

 anchoring a boat, or other direct method. 



Specific Gravity of Sea Water. — The specific gravity of the sea water during 

 the voyage was ascertained by Mr. Buchanan, the analytical chemist of the Expedition, 

 with a delicate hydrometer, designed by him for this purpose, which is described in 

 Physics and Chemistry, vol. i. 



The most cursory glance suffices to show that the varying phenomena with which 

 meteorology deals are all referable to the sun, it being evident that if the sun were 

 blotted out from the sky, a cold lifeless uniformity would take possession of the whole 

 surface of the earth. It is thus that all meteorological phenomena may be conveniently 

 grouped into two great classes : those resulting from the revolution of the earth on its 

 axis, and those resulting from its revolution round the sun taken in connection with 

 the inclination of its axis to the plane of its orbit. Hence meteorology falls into two 

 great divisions, the first embracing diurnal and the second annual phenomena. 



Humboldt, Dove, Kuppfer, Quetelet, Sabine, Brewster, and the other early 

 founders of the science, were keenly alive to the essential importance of hourly observa- 

 tions in the investigation of diurnal phenomena. By the influence they brought to 

 bear on the Governments of the day, meteorological and magnetical Observatories of the 

 first class were established widely over the civilised world, from which observational 

 data were obtained that will always hold their place as contributions of the first 

 importance to science. 



On the basis of these observations the diurnal changes in atmospheric pressure, 

 temperature, humidity, and wind were partially revealed and explained. It came, 

 however, by and by to be felt that data supplied exclusively by Observatories on land 

 were inadecpiate to a right conception and explanation of meteorological phenomena, 

 and accordingly when the Challenger Expedition was fitted out, arrangements were 

 made for taking, during the cruise, either hourly or two-hourly meteorological observa- 



