Mat 25, 1S83.] 



SCIENCE. 



Table of rainfall at Panama and other stations. 



453 



accompanies the curve of maximum daily tempera- 

 ture due to tlie annual oscillatory movement of the 

 thermal equator. The movement of this curve is 

 closely connected with the annual movement of the 

 sun across the geographical equator. The sun passes 

 the zenith of the isthmus at mid-day twice in the year, 

 on April 13 and Aug. 29. The sheet covers tlie isth- 

 mus from the heginning of May to the end of June, 

 and from the end of July to the heginning of Decem- 

 ber. These two intervals occurring between the first 

 of May and tlie first of December constitute the rainy 

 seasons. The first is generally interrupted by the short 

 ' summer of St. John.' During the remainder of the 

 year is the dry season. At this time the sheet is en- 

 tirely to the south of tlie istlimiis, while during the 

 ' summer of St. John ' it is entirely to the north. 



On the north side of this sheet the trade-winds of 

 the nortliern hemispliere prevail, wliicli, at the istli- 

 mus, have in general a direction from the north-east. 

 On the south side the trades of the southern hemis- 

 phere prevail, which have a direction from the south. 

 In the interior of the sheet, at the earth's surface, 

 tlie wind is feeble and uncertain. This, then, for the 

 isthmus, is the period of calms, the time of gentle 

 breezes; now from the land, now from the sea, ac- 

 cording to the liour of the day. 



M. de Lesseps further remarks, that one can see, 

 that, in the time during which the (overhanging) 

 sheet of ascending air is over the istlimus, the season 

 of rain prevails, because tlie trade-winds, blowing 

 along the ocean's surface, accumulate in this sheet a 



mass of vapor, which rises up, comes to the higher re- 

 gions of the atmosphere into lower and lower temper- 

 atures, and is condensed ; producing, thus, a vault of 

 perpetual cloud, which generally surrounds tlie earth 

 in a dark ring, — called, by the French sailors, ' pol 

 an noir; ' by the Americans and English, ' cloud 

 ring,' — and continually precipitates during the rainy 

 season the showers of the tropical regions. 



The waters of the gulf-stream which come from the 

 equator are charged with a great quantity of vapor; 

 and tliis is condensed and precipitated by the Cor- 

 dilleras. This accounts for the abundant rains of the 

 Atlantic watershed. Tliis cause does not exist on 

 the Pacific waterslied. The general current along the 

 coast of the isthmus is just the reverse of that in tlie 

 sea of the Antilles. On the contrary, the tide comes 

 from the nortli; and in consequence these waters are 

 cooler, and furnish less vapor to the air flowing along 

 the surface. This explains why it rains more at 

 Colon than at Panama, and why, in proportion as one 

 removes from the Atlantic coast, the rain diminislies. 

 So upon the island of Naos, situated in tlie Bay of 

 Panama; and, where the canal company has estab- 

 lished a meteorological station, the rain gathered is 

 less than at Panama. 



The existence of winter and summer rains in belts 

 approximately parallel to tlie equator has been long 

 recognized. A glance at tlie table above will show 

 that the rains all along the Pacific coast are markedly 

 periodic, and occur later in the year as we go north; 

 and the heavier rainfall occurs at the time the sun 

 is the farthest south of the equator. 



H. A. Hazen. 



THE COPPER-BEARING SERIES OF 

 LAKE SUPERIOR. 



It may not be unprofitable, at this presumably the 

 closing stage of the present discussion of the Ke- 

 weenawan series, to state summ arily the main grounds 

 on which its pre-Potsdam age is maintained. It is 

 obvious that such a statement can but imperfectly 

 indicate the nature of the evidence relied upon; for 

 the significant data are derived from numerous lo- 

 calities, and from diverse phenomena which cannot 

 be adequately, and at the same time briefly, described. 

 The formation involves an area of upwards of forty 

 thousand square miles ; and only a wide survey of it, a 

 critical elaboration of trustworthy observations, and a 

 judicial treatment of the evidence, can command com- 

 plete deference, and that is a thing of the future. No 



