ADVANCEMENT OE SCIENCE. 



127 



both anchors ahead, and was only brought up by paying out nearly all her cable. 

 The heights of the tides and levels of the two oceans have been well established 

 by the recent observations of Col. Totten in Navy Bay on the Atlantic and in a deep 

 bend of the Bay of Panama on the Pacific. On the Atlantic a consecutive series 

 of thirty-two observations were taken in the months of August and September 

 during the season of calms. On the Pacific two sets of observations were made. 

 The first, during May and June, when fifty-four consecutive tides were observed 

 in a season of calms : and the second in November and December, when fifty -two 

 consecutive tides were observed in a season of light winds. The results do not 

 exactly correspond, and are given in the following table : — 





Pacific. 



Atlantic. 





May and 



Nov. and 



Aug. and 





June. 



Dec. 



Sept. 



Greatest rise of tide 



tf-12 



21.30 



1-60 



Least 



1-94 



9-10 



0-63 



Average 



12-08 



14-10 



1-16 



Mean tide of Pacific above mean tide of Atlantic. . 



0-759 



0-140 





High spring tide of Pacific above high spring tide 









of Atlantic 



9.40 



1012 





Low spring tide of Pacific below low spring tide of 









Atlantic 



6-65 



9-40 





Mean high tide of Pacific above mean high tide of 









Atlantic 



6 25 



6-1S 





Mean low tide of Pacific below mean low tide of 









Atlantic 



4-73 



5-26 





Average rise of spring tides 



14-08 



17-30 





Average rise of neap tides 



9-60 



12-40 





These observations make the mean level of the Pacific from 0-14 to -f 75 higher 

 than the mean level of the Atlantic ; but this is probably owing only to local circum- 

 stances, and it may be assumed that there is no difference in the mean levels of the 

 two oceans. The conclusions arrived at by the successive independent surveys 

 carried out at the expense of Mr. Selley may be summed up as follows : — First, 

 That the oceans can be united through the Atrato and Truando by a canal with- 

 out a lock or any other impediment. Second, That while the distance between the 

 oceans by this route is only 131 miles, half that distance is provided by nature with 

 a passage for the largest ships. Third, The remaining distance requires the re- 

 moval of bars, excavations, and cuttings, presenting no unusual difficulties. Fourth, 

 Harbours, requiring but lktle improvement to render them excellent, exist at the 

 termini. 



ON ISOTHERMAL LINES, BY PROFESSOR HENNESST. 



The author discusses the distribution of those lines in islands. Considering an 

 island having its shores bathed by a warm oceanic current, the isothermals, if the 

 direct solar radiation were abstracted,, would be closed curves surrounding the 

 centre of the island and related to the coast-line, their shapes being variously 

 modified by ranges of mountains, inequalities in the surface, and prevalent winds. 

 By now introducing the effect of solar radiation it follows from the mathematica 

 theory of heat, that the entire quantity of heat received by a unit of surface of 



