170 FISIIIXG-GROUNDS OV NORTU AMERICA. 



tbeniial Hues are coustructeil for every five degrees of tempeiatiire from 40° to 80°, aud are carried 

 vertically from statiou to station, connecting the dates at which the temperatures they represent 

 were reached at each station ; the data for each separate statiou are to be read across the chart 

 from left to right. Two series of isotherms are actually included on each chart, one relating to the 

 l)oriod of rising temi)eratures in the first half of the year, the other to that of falling temperatures 

 in the hist half of the year. The space included between any two isotherms of equal value is sup- 

 posed to represent a period during which the temperature was always equal to or above that indi- 

 cated by the isotherms. Those portions of the isothermal lines consisting of dashes denote the 

 lack of observations for the stations opposite them. Complete breaks in the lines generally indi- 

 cate that the temperature did not reacli the isotherm at that station during the year, or during the 

 period of either rising or falling temperature. 



In explanation of the arrangement, reference may be made to the isothermal chart for 1881 

 (No. 26). In that year the isotherm of 40° did not extend south of Body's Island, where the tem- 

 perature reached 4()o about the middle of February. At Winter Quarter Shoal the same tempera- 

 ture was reached March 20; at Five-Fathom Bank, April 15 ; at Absecon Inlet, March 20; at Sandy 

 Hook, April 14; at Fire Island, April 8; at Block Island, April 16. The temperature remained 

 above 40° throughout the rest of the year, and until after January, 1882, at Body's Island, Winter 

 Quarter Shoal, Five Fathom Bank, and Sandy Hook; until January 2, 1882, at Absecon Inlet ; 

 until December 31, at Fire Island ; and until January 1, at Block Island. The isotherms of 4.5°, 50°, 

 55°, &c., are reached at successively later dates during the period of rising temperature, and at 

 earlier dates during the period of falling temperature, Init the intervals between them vary greatly 

 at the different stations. 



RANGES OF THE ISOTHERMS. 



A detailed comparison of tiie isothermal charts would tend to confuse rather than to aid refer- 

 ence to them, and our remarks on the subject will be limited to a few statements respecting the 

 range and general position of the isotherms. 



The isotherms of 40° and 4.5° are generally co-extensive iu their range. They always reach 

 as far north as Petit Manan, and frequently as far south as Body's Island, but may stop at either 

 Winter Quarter Shoal or Five-Fathom Bank; in 1882, the isotherm of 40° extended south only as 

 far as Absecon Inlet. The isotherm of 50° begins at the north at Petit Manan, and at the south 

 may terminate at Cape Lookout or Martin's Industry. The isotherms of 55° to 70°, inclusive, 

 always reach south to Martin's Industry, but no farther; while those of 75° and 80° are the only 

 ones ranging aloyg the Florida Reef stations to the Tortugas. A temperature of 55° is often 

 recorded at Petit Manan, but seldom at the next two stations to the westward— Mount Desert 

 Eock aud JIatinicus Rock. South of here the isotherm of 55° is generally continuous. The 

 temperature usually reaches 00° at Boon Island and Thatcher's Island (although at the latter 

 station observations are wanting for 1884 aud ISSo), but during some years remains lower than 

 this at Pollock Rip and Nantucket New South Shoal. The isotherm of 65° generally extends 

 northward to Vineyard Sound, but in 1884 it began at Breuton's Reef; that of 70° extends north 

 to Fire Island or Block Island. Absecon Inlet is the northern limit of the isotherm of 75°, which 

 in some years, however, does not reach north of Body's Island or Cape Lookout. The isotherm of 

 80° does not pass north of Body's Island. 



CHANGES IN POSITION OF TIIE ISOTIIEliM.y I'M DIFFERENT YEARS. 



During the five years represented by the charts there is considerable change in the positions 

 of the isotherms of equal value from year to year, frequently amounting to a month in time, and 



