Page:The American Cyclopædia (1879) Volume II.djvu/86

 ATLANTIC OCEAN Gulf stream of the latter. It would take too much space to detail all the theories of modern authors, but a few must still be mentioned. Capt. M. F. Maury gave an exaggerated weight to differences of density of sea water in north- ern and southern parts of the ocean. Sir John Herschel, in his article on physical geography in the " Encyclopsedia Britannica," attributed the currents to the effect of the trade winds. Before his death he seems to have fallen in with the views of Prof. Carpenter mentioned under the head of Gulf stream. Dr. Muhri of Gottingen, in his work on ocean currents, gives the following conclusions: 1. There are in ocean circulation two great movements per- pendicular to each other, the one following the equator, the other the direction of the me- ridians. 2. The equatorial circulation results from the inertia of water with regard to the rotation of the earth ; the meridional or ther- mometric circulation is caused by the difference of temperature between the polar and equato- rial regions. 3. The meridional as well as the equatorial circulations exhibit two motions in contrary directions, which compensate each other and are superposed to each other in part in the thermometric circulation, on account of their unequal density. 4. The unequal distri- bution of the continents impedes the regularity of the great movements of circulation, and, in conjunction with the unequal relief of the bot- tom and the action of the winds, induces sec- ondary currents disturbing the general motion. Gulf stream. The importance of this great current to the commerce and navigation of North America, to which reference has been made before, the great scientific interest it pre- sents by its size, temperature, and influence on climate, have made it, in the words of Prof. Bache, " the great hydrographic feature of the United States coast." Under the superinten- dence of the late Prof. Bache, the United States coast survey has accumulated a large number of observations of that part of the stream comprised between its entrance into the straits of Florida and the region where it leaves the coast after having changed its course to the east. The observations were directed chiefly toward the determination of the depth, the figure and con- stitution of the bottom, and the temperature from the surface down through the whole depth. The instruments used for temperature have been of various construction. Metallic thermometers in the watch form were used, enclosed in strong brass vessels ; they answered well enough, and were employed to a consider- able extent in the earlier researches ; but in several instances the brass box was crushed by the pressure. Self-registering thermometers in glass globes were used also, but they had the inconvenience of experiencing the changes of temperature too slowly. Six's self-register- ing thermometers were used extensively, up to about 100 fathoms, beyond which they are liable to be crushed ; and in all cases their in- dications are rendered very erroneous by the pressure. For great depths Saxton's metallic thermometer has been of great service. This instrument consists in a ribbon of two metals of different expansion, soldered together and ! rolled in a cylindrical spiral around a spindle, j to which the movement of expansion or con- traction is communicated, and by it transferred to a hand or 'needle moving an index over a graduated dial. The whole is enclosed in a suit- able case perforated for the passage of the water. It works well, but is affected by pressure in a manner not easily explained. At present the Miller-Casella protected thermometer is used, and proves an excellent and trustworthy instru- ment. It is in the main a Six's self-registering maximum and minimum thermometer, the bulb of which is protected from pressure by an outer bulb blown over it and sealed round the neck, a space being left between the two bulbs, par- tially filled with alcohol, in order to communi- cate the temperature more rapidly to the inner bulb. The observations were made at a num- ber of stations in lines or sections at right angles to the stream. The thermometer was observed at the surface and at different depths, generally at every ten fathoms as far as 50, and at every hundred fathoms in greater depths. When the change of temperature was very rapid, the number of sections, stations, and observations was multiplied to keep pace with it. The re- sults were arranged afterward in diagrams, where the changes of temperature were repre- sented by curves, thus giving at a glance the distribution of heat throughout the stream. From these observations the following general deductions were made : In the sections between Florida and Cuba the highest temperatures were found near the Cuban coast, where also the greatest depth was recorded. It was ob- served by Mr. Mitchell that very near the coast of that island the stream had a uniform velocity and constant course for a depth of 600 fathoms, although in this depth the temperature varied 40. The stratum of warm water was found to be of much greater thickness or depth toward the middle of the straits than nearer shore; thus at a distance of 6 or 7 m. from Havana the layer of water above the tempera- ture of 70 extended only to a depth of about 70 fathoms, while some 30 m. off the co.'ist its thickness was about 180 fathoms. The slope of the bottom is very abrupt on the Cuban coast, but much more gradual on the Florida side, where the current is also more irregular, taking sometimes even the shape of a counter current running west. It is also here affected by the winds and tides. The same character as in this section is maintained throughout the straits of Florida to the narrows of Bernini. No permanent current was found j in the St. Nicholas and Santarest channels, I sometimes regarded as partial feeders of the Gulf stream. Toward the narrows of Bernini the breadth and depth of the straits diminish and reach their minimum, the breadth being only 44 m. and the greatest depth 370 fathoms.