INVESTIGATION OF BALTIC SEA AND GERMAN OCEAN. 293 



The difference of 29^ in the temperature of the atmosphere between the 

 coldest and the warmest month is lowered to 24o.5 in the surface-water, 

 and at a depth of 4^ fathoms to 24P. The absolute extremes, however, 

 are as follows : In the air, 57C'.6 F. ; in the surface-water, 26°.!, and in 

 deep water, 24P.3. The average volume of the heat of the water in all 

 strata exceeds that of the heat of the air, and it is therefore probable 

 for Heligoland that a source of heat must be looked for in the entering 

 heavier under-current of ocean water. Although the temperature of the 

 air falls considerably below the freezing-point, the temperature of the 

 water at Heligoland in all the strata remains above this point, and, 

 without doubt, this difference causes one of the principal differences in 

 the development of organisms which cannot be explained by the differ- 

 ences in the percentage of salt. 



For further information the reader is referred to the report of the 

 Commission. 



AUXILIARY APPARATUS. 



For the various observations many auxiliary implements are required, 

 such as lead-lines, hoisting-apparatus, sounding-cups, water-bottles, &c. 

 A few words may be said in relation to these, since the usefulness 

 of areometric determinations depends upon the well-working of the 

 machinery. 



To secure specimens of bottom water a simple arrangement, first used 

 by Dr. H. A. Mayer, is employed. A strong, well-corked flask is lowered 

 to the desired depth, when it is uncorked by a sudden jerk of the line; 

 the drawing up may be easily done, as experience has shown, without 

 a noticeable change in the quality of the water. This arrangement, 

 however, cannot be used for any but moderate depths, having, like all 

 other means for this purpose, the disadvantage of not permitting a gas- 

 analysis, the air of the bottle becoming partially absorbed by the enter- 

 ing water. An examination of waters thus obtained shows the presence 

 of irregular quantities of the permanent gases, partially derived from 

 the air of the bottle. With the view of exact areometric determinations 

 and gas analyses the Commission has made use of various instruments, 

 those of Professor Dr. Jacobsen, in Eostock, and of Dr. H. A. Mayer 

 proving the best. The apparatus of Jacobsen consists of an India-rub- 

 ber bag partially iilled with mercury and freed from air by pressure. 

 The cork is self-regulating, opening as soon as the bottom is reached 

 and closing again when the bag is drawn up. The apparatus of Mayer 

 consists of a wide, open metallic cylinder, with bottom valves, and per- 

 mits the bringing up of water from any depth. For further details, 

 especially regarding the amount of carbonic acid, reference is made to 

 the annual reports of the Commission. 



Of the other apparatus used during the investigation we only mention 

 the current-meter. The instruments permitting of determination of the 

 direction and intensity of the currents are very defective. From an 



