622 



Dr. W. B. Carpenter on the 



[June 13, 



the superheated layer is well shown in the contrast between the Mediterra- 

 nean and the portion of the Gulf-stream that lies between the same parallels. 

 For while the surface-temperature of the former sometimes rises as high as 

 that of the latter, it is subject to a considerable diurnal range, in conse- 

 quence of the thinness of its superheated stratum ; while its seasonal range 

 closely follows the temperature of the atmosphere, down to its constant 

 winter mean of about 54° (Report for 1870, § 89). But in the latter 

 there is scarcely any diurnal range ; while its winter temperature only falls 

 to 72° off Cape Hatteras, and to 67° off Nantucket ; for as a temperature 

 of 60° or upwards extends to a depth of from 200 to 300 fathoms, the 

 whole of this sub-surface stratum must be reduced to that standard, or 

 below it, before the <swr/«ce-temperature would fall to 60°. — It follows as a 

 Corollary, that a deep stratum having a moderate excess of temperature 

 may be a far more effectual carrier of Heat into a colder area than a much 

 warmer superficial flow. 



138. The thermal power of the Gulf-stream, or of any similar Current, 

 must depend on three factors, — (1) the Sectional Area of the Stream; 

 (2) the Rate of its flow ; and (3) its Temperature. 



139. It is impossible, in the present state of our knowledge, to arrive at 

 any exact estimate of the sectional area of the Stream ; since it is for the 

 most part only from the Temperatures of its different strata that we can 

 judge whether they are, or are not, in movement, and what is the direction 

 of their movement. If we accept the conclusion of the U.S. Surveyors 

 (§ 129) that in the Havana Section the depth of the outward current was 

 not above one-third of the greatest depth of the Channel, — namely, about 

 200 fathoms, — and take the average temperature at that depth to be about 

 60°, we may assume that the stratum extending downwards to 60° in other 

 sections represents the real outwardly flowing Stream. Now it will be 

 seen that in the Cape Florida and Bernini Section, the line of 60° in the 

 channel of the Gulf-stream proper has an average depth of about 175 fathoms, 

 or 1050 feet ; but the breadth of the outflowing Stream cannot be taken 

 (according to Prof. Bache's estimate) at more than 25 miles. — Mr. Croll's 

 estimate of the quantity of water as equal to a stream 50 miles broad 

 and 1000 feet deep, appears to be based on the sectional area of the entire 

 channel, which he states at 30 miles broad and 1950 feet deep ; but it seems 

 clear that the lower portion of this channel must be occupied by an in- 

 flowing current ; and the only question is with respect to the depth at 

 which the reversal of the flow takes place. 



140. The mean annual rate of the Gulf-stream current is estimated by 

 our Meteorological Department, on the basis of a large body of observations 

 taken at various periods of the year, at not more than two miles per hour, 

 or 48 miles per day, even in the " Narrows " where the current is most 

 rapid. The rate stated by Mr. Croll, namely four miles per hour, is only 

 seen at the period of the greatest strength of the current. — But the rate 

 given by swr/ace-observalions affords no indication whatever of the rate o 



