EXPLANATION OF OCEAN TEMPERATURE CHART No. 20. 



THATCHER'S ISLAND LIGHTS, MASSACHUSETTS. 

 Obseri-ers: E. C. Goss, O. B. Colk, 6. L.\kbmann, oft lie U. S. Signal Service. 



Location of station. — Tlie Cape Ann lights are located on Thatcher's Island, about three-fourths of a mile off the 

 eastern extremity of Cape Ann, both being on the outer side of the island. There are several rocky ledges in front of 

 the island, but otherwise depths of SJ to 24 fathoms occur within a di.stauccof 1 mile. A depth of GO fathoms is reached 

 5J mi^s to the eastward. Thatcher's Island is about 73 miles northwesterly from Pollock Rip light-ship. 



Geographical position. — The northern light is located in latitude 42° 38' ai" N. ; longitude, 70° 34 31" W. 



Depth of water. — Seven feet. 



liangc of temperature (twelve months).— Air, 480.5 (30° to 78°.5) ; surface, 32° (35° to C7°). 



The observations at this station were taken by trained observers of the Signal Service, and the winter records, 

 although covering only two years, are presumably accurate, and have been plotted. Observations were continued 

 through only three years, and there are many gaps within that period, which is very unfortunate, considering the 

 important geographical position of the station. Only one observation was taken daily, at 2 p. m. The surface curves 

 are very irregular, and in many cases, especially during the warmer months, indicate direct atmospheric influence, 

 from the similarity of the variations in bolh the air and surface curves. There is not always, however, a strict 

 correspondence in the relative positions of the curves in different years, the air curve from May to July, 1883, 

 being from 2° to 10° higher than the air curve for (lie same months in 18S1, while the surface curve for a part of the 

 same period, in 1S8.^, is from 2° to G° lower than in 1381. During the colder months the surface curves are very 

 regular. 



The maximum air temperature is higher at Thatcher's Island than at any of the preceding stations as far soul h as 

 Block Island, with which latter this station corresponds approximately. In surface temperatures Thatcher's Island 

 agrees most closely with Vineyard Sound and Brentou's Reef light-ships, but it is probable that the higher surface 

 ^.emperatnres of Thatcher's Island are due to the observations having been taken in a sheltered place. 



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