OCEAN TEMPERATURES. 173 



Reef, at the foimer statiou occuniiif;' between Augiif^t 1 and September 20, and at the latter be- 

 tween October 20 and December 8. 



Eighty-five degrees. — The isnrfaee waters seldom reach a teinperatureofSyC excepting for 

 short periods at the extreme south, and it has been impossible to coustrnct isotherms for more 

 than one j'ear at Martin's Industry and the Florida Reef stations. Temperatures of 85^ and over 

 •were recorded between the Sth and last of July, 1881, at Rattlesnake Shoal, Fowey Rocks, and 

 the Tortugas; between the lirst part of July and the last of August, 1883, at (Jarysfort Reef and 

 Fowey Rocks ; and in 1885, from August 2 to 2G, at Martin's Industry, from July 29 to Septendjer 

 28, at Fowey Rocks, and from July 9 to September 25, at Carysfort Reef. 



Ninety degrees. — A surface temperature of 90° was occasionally recorded at some of the 

 extreme southern stations, but never for more than a day or two at a time. 



MEAN ISOTDERMS BASED IPON I'lTE YEARS' OBSERVATIONS. 



lu constructing chart No. 31, it was impossible, in all cases, to obtain the reductions of five 

 years' observations, ou account of occasional breaks in the records, but the number of years 

 plotted is never less than three and generally more than four. The exact number iu each instance 

 may be determined by reference to the annual isothermal charts (Nos. 2G-30). 



Ou this chart the surface isotherms are represented as being much more regular and more 

 uniformly distributed with reference to time than ou any of the yearly charts. Ou the left-hand 

 side of the chart, north of Body's Island, there is a wide area of low temperatures, bounded by the 

 isotherms of 40°. From Winter Quarter Shoal to Fire Island, inclusive, this area represents a 

 period of about seventy-five to one hundred days, being shortest at Five-Fathom Bank, and 

 longest at Sandy Hook ; from Block Island northward the length of this period varies from one 

 hundred to one hundred and thirty-five days, being shortest at Brenton's Reef and longest at Boon 

 Island. A narrower space of maximum temperatures extends vertically through the center of the 

 chart, occupying principally the month of xVugust, which is not crossed by any of the isotherms, 

 although a few of them extend a short distance into it. The length of time elapsing between 

 successive isotherms is generally from about twelve to twenty-four days, seldom less but often more. 

 The isotherms are more numerous and follow one another more ra[)idly at the intermediate stations 

 of the series than at the northern and southern stations ; toward the north and south they diverge 

 somewhat and become more widely separated. This naturally results from the fact that at the 

 intermediate stations there is a much greater range of tem])erature (above a minimum of 40°) than 

 at the northern and southern ones. In the eastern part of the Gulf of Elaine there are only three 

 continuous isotherms of the value of those i)lotted (40^ to 50°, inclusive); in the western part four 

 such isotherms (40° to 55°, inclusive). Ou the southern coast of New England there are five 

 isotherms at Nantucket New South Shoal (40° to 00°, inclusive;, and six from Vineyard Sound to 

 Block Island (40° to 05°, inclusive) ; seven extend from Fire Island to Body's Island (40° to 70°, 

 inclusive) ; four occur on the coast of South Carolina (55° to 70°, inclusive), and two at the Florida 

 Reefs (75° to 80°, inclusive, and sometimes 85°). 



North of Body's Island the isotherms of both series extend in a slightly oblique direction trend- 

 ing inward toward the north. At Cape Ilatteras they bend abruptly, and the same isotherms 

 appear much earlier iu the year to the south of that important cape. 



At Frying Pan Shoals, the first reliable statiou south of Ilatteras, the isotherm of :)3° api)ears 

 about ninety days earlier than at Winter Quarter Shoal; the isotherm of GOo about sixty days 

 earlier; the isotherm of 05° about fifty days earlier; and the i.sotherm of 70° also about fifty days 

 earlier. 



