TRANSACTIONS OF THE SECTIONS. 171 



have chavmed so many readers, and in their practical bearings have uudouhtedly 

 advanced the practice of navigation. 



In our admiration, however, of modern progress we must not in justice pass by 

 withoiit recognition the labours of earlier workers in the same field. So early as 

 the middle of the seventeenth century we find, in Holland, Barnard Vanerius 

 describing with commendable accuracy the direction of the greater currents of the 

 Atlantic Ocean and their dependence on prevailing winds — the unequal saltness 

 of the sea, the diversity of temperature, as tlie causes of the direction of the winds— 

 and also speculating on the depths of the sea. Vanerius's geographical writings 

 were highly appreciated by Newton ; and editions were prepared at Cambrid''^e 

 under the supervision of that great man in 1672 and 1081. " 



_ To Dampier, the seaman, and Halley, the philosopher, we owe graphic descrip- 

 tions of the trade-winds as derived from personal experience ; wliile their causes 

 were investigated by lladley, and the conclusion he arrived at, that they were 

 due to the combined eftocts of the diurnal revolution of the earth on its axis and 

 the unequal distribution of heat over diflerent parts of the earth's surface, in 

 substance still remains unchallenged. 



To Eennell we owe a masterly investigation of the currents of the Atlantic 

 Ocean, an investigation which for precision and a thorough conception of the con- 

 ditions affecting the subject will long serve as a model for imitation. His period 

 covered some thirty or forty years during the end of the last and the beginning of 

 the present century. At that epoch chronometers, though very efficient, had 

 scarcely passed the stage of trial, but had nevertheless commended themselves to 

 the first navigators of the day, whose aim it was to narrowly watch and test 

 this, to them, marvellous acquisition. Eennell thus commanded nautical observa- 

 tions of a high order of merit ; these he individually verified, both for determining 

 the ship's position absolutely and relatively to tlie course pursued ; and our know- 

 ledge of surface-currents was established on the secure basis of differential results 

 obtained at short intervals, such as a day or parts of a day, instead of the previous 

 rude estimation fl-om a ship's reckoning extending over a whole vovage, or its 

 greater part. 



At a later date we have, by Eedfield, Eeed, Thom, and others, solidly practical 

 investigations of the gyratory and at the same time bodily progressive movements 

 of those fierce_ and violent storms which, generated in tropical zones, traverse 

 extensive districts of the ocean, not unfrequently devastating the narrow belt of 

 land comprised in their track, and on the sea baiffling all the care and skill of the 

 seaman to preserve his ship scatliless ; while the clear and elegant exposition by 

 Dove of their law and its application as one common general principle to the 

 ordinary movements of the atmosphere must commend itself as one of the achieve- 

 ments of modern science. 



While for the moment in the aerial regions, we must not forget the industry 

 and scientific penetration of the present excellent secretary of the Scottish 

 Meteorological Society. His more recent development of the several areas of 

 barometric pressure, both oceanic and continental, bids fair to amend and enlarge 

 our conceptions of the circulation of both the aerial and liquid coverino-s of our 

 planet. ° 



Looking, then, from our immediate stand-point, on the extent of our Imowledge 

 (as confinnedby observational facts) of the several branches of phvsics pertaining to 

 the geography of the sea, just rapidly reviewed, we find that, resulting from ''the 

 methodical gathering up of "ocean statistics " by our own and other maritime 

 nations, in the manner shadowed forth by Maury and stamped by Ihe Brussels 

 Conference of 1853, we are in possession of a goodly array of broad but neverthe- 

 less sound results. The average seasonal limits of the trade-winds and monsoons, 

 with the areas traversed by circular storms, are known, also the general linear 

 direction and varying rates of motion of the several ocean currents and streams, 

 together with the diflused values of air and sea-surface tempera tiues, the areas of 

 uniform barometric pressure, and the prevalent winds, over the navigable parts of 

 the globe. 



Thus far the practical advantages that have accrued to the art of navigation 

 (and so directly aiding commerce) by the gradual diffusion of this knowledge, 



