3*4 



NA TURE 



[Ju/y 31, 1884 



In the course of his work Prof. Dittmar hit upon an 

 elegant method of determining densities, which was found 

 to be very satisfactory. He filled a water-bath with a 

 particular sea-water that had been selected as a standard, 

 and kept it at a constant temperature. A specific-gravity 

 bottle was filled with the same water, stoppered, hung in 

 the bath to the balance-pan, and weighed accurately. To 

 compare any number of samples of water it was sufficient 

 to fill up the bottle with the water in question, and again 

 weigh it immersed in the standard. Prof. Dittmar con- 

 fined himself to making out the relation between salinity, 

 specific gravity, and temperature, leaving the relation of 

 specific gravity and pressure for subsequent treatment by 

 Prof. Tait, whose great chain of experiments on the com- 

 pressibility of sea-water is now drawing to a close, and 

 the results of which will shortly be published in the 

 Challenger Reports. The conclusion Prof. Dittmar comes 

 to is summed up in the formula — 

 ,S, - ,W, 

 A a + bl + e t* 

 where x is the " salinity" or permilleage of halogen ; 4 S, the 

 specific gravity of sea-water at t° relatively to pure water 

 at 4° C. ; ,W, is the specific gravity of pure water in the 

 same way ; a, b, and c are constants which have been 

 determined once for all. 



In the chapter on Salinity in Part I. Prof. Dittmar gives 

 a table dealing with 300 samples, collected in all parts of 

 the ocean and from all depths. These tables show the 

 position, the depth of the ocean at the station, the depth 

 from which the sample was drawn, the permilleage of 

 chlorine (x), the mean deviation of the mean x from the 

 individual results, and the difference between the amount 

 of chlorine as calculated from Buchanan's observations of 

 specific gravity, and as found directly by Dittmar. 



Mr. Buchanan gives in his report all his observations 

 classified according to geographical position and depth, 

 and arranged in eighteen large tables. These record the 

 specific gravity of water from all depths in the North and 

 South Atlantic, the Southern Indian Ocean, the North 

 and South Pacific, and the interesting inclosed seas of 

 the Malayan Archipelago. The numbers are simply 

 given as they were observed, only corrected for tem- 

 perature by Dittmar's table, and all discussion of their 

 oceanograpnic significance has been deferred until a 

 subsequent occasion. 



A series of coloured charts illustrating the bathy- 

 metrical and geographical distribution of specific gravity 

 over the whole world and in the individual oceans ac- 

 companies the memoir. These are extremely interesting, 

 and in many cases they tell their own story without ex- 

 planation, though when the full descriptions are published, 

 the value and interest of the plates will be greatly increased. 

 The track-chart of the Challenge?- coloured to show the 

 surface salinity of the ocean, is especially worthy of 

 notice ; its details have been filled in, and the whole ren- 

 dered more complete, by the incorporation of the results 

 of other exploring expeditions. 



The great importance of Mr. Buchanan's specific gravitj 

 observations will be more readily recognised by the general 

 reader when they are elucidated by the work of Prof. Tait 

 and Mr. Buchan, and treated more generally than is pos- 

 sible in a mere statement of observed figures. 



The third part of the volume is devoted to the tem- 

 perature observations made during the cruise. The 

 nature of the information contained in the curves 

 which make up this part of the work is very concisely 

 put in the editor's introduction : — 



"It has been deemed advisable to publish, for the 

 convenience of scientific men, the whole of the deep-sea 

 observations of temperature made during the voyage of 

 the Challenger. These are given in detail in the accom- 

 panying series of 263 plates, which show the latitude and 



longitude of the station ; the depth in fathoms of ths 

 bottom ; the depth at which each temperature was taken ; 

 the number of the thermometer ; the temperature actually 

 observed read to quarter degrees ; the error of the thermo- 

 meter ; and the temperature corrected for instrumental 

 error only." 



The temperatures have been plotted by Staff-Com- 

 mander Tizard, and a free-hand curve drawn through the 

 points. From this the " temperature by curve " which is 

 employed in drawing the diagrammatic sections of the 

 ocean showing the bathymetrical arrangement of the iso- 

 thermals is taken. These sections will' be published in 

 vol. i. of the Challenger narrative, and to the general 

 reader they will present a much more intelligible idea 

 of the distribution of oceanic temperature than can 

 be given by the study of tables of figures or curves for 

 separate stations. The separate station curves are, how- 

 ever, of the utmost value to any one who wishes to make 

 a detailed study of ocean temperatures. With a direct 

 view to such a purpose the curves have been drawn with 

 rigid adherence to the numbers in the observation books, 

 even the most obvious cases of observational error being 

 left uncorrected ; for the specialist can easily discover and 

 correct them himself, and no one else will notice them. 



The temperature observations, like the specific gravity 

 observations, form a rich mine of material with which 

 good work may be done. It is shown by a glance at the 

 charts that there are areas in the ocean of great salinity 

 and areas of great dilution ; it is shown that the pressure 

 increases uniformly with the depth ; it is known that the 

 surface temperature of the water varies greatly in different 

 latitudes, and that, as the depth increases, the tempera- 

 ture decreases, at first very rapidly, but after the first 

 few hundred fathoms with increasing slowness, until at 

 the bottom the temperature of the open ocean is every- 

 where the same, between 34° and 35° F. ; it is known also 

 that in inclosed seas, or in those where there are sub- 

 marine barriers cutting them off from the rest of the 

 ocean, the temperature assumes a constant value in its 

 descent, and sometimes the bottom is nearly 20 F. 

 warmer than that of the ocean at the same depth a few 

 miles distant ; but this is all that is known. It is evident that 

 there must be an ocean circulation on a magnificent scale 

 going on, a gradual onward sweep of the whole mass of 

 the water, but the direction of this mass motion can only 

 as yet be guessed at, and its rate is utterly unknown. The 

 material for solving this, the great oceanographic problem, 

 is rapidly accumulating, and when the physical and 

 chemical reports of the Challenger Expedition have all 

 been made public, it will be strange indeed if a large 

 generalisation cannot be based upon them, and the dis- 

 covery of the secret of ocean circulation be added to the 

 many discoveries which have been made by the scientific 

 men of the cruise. 



The nature of this volume, both on account of the sub- 

 jects with which it deals and the number of formula- and 

 long tables of numbers it contains, must have made the 

 task of editing it no light one ; and the accuracy of every 

 part, the almost entire absence of typographical errors, 

 and the beauty of the lithographed charts show that 

 authors, editor, engravers, and printers have alike exerted 

 themselves to produce a volume worthy of being the first 

 to record the physical and chemical work of the Chal- 

 lenger Expedition. Hugh Robert Mill 



SPECIALISATION IN SCIENTIFIC STUDY^ 

 '"PHERE once was a science called "natural philo- 

 *■ sophy," which, like some old synthetic types of 

 animals, held in itself all the learning that applied to 

 physical facts. By the beginning of this century this 

 science of natural things had become divided into physics 

 and natural history. These divisions have since spread, 



