March 4, 1887.] 



SCIJEJSrCE. 



205 



Referring again to the diagram, it will be seen 

 that the two lunes, or spaces between the type- 

 curves, are equal, i.e., the slopes creating the 

 flood-current and the ebb-current, alternately, are 

 equal in anaount and duration. There is the same 

 ' head ' for one stream as for the other. But 

 there is a very important difference in the positions 

 of these lunes, which affects materially the rela- 

 tive values of the slopes they represent. I have 

 marked with a cross the centre of each, and it 

 will be observed that the right-hand lune is one 

 foot above the other, — which means that the ebb 

 (westerly) current is in deeper water and greater 

 transverse section than the flood. It is, therefore, 

 the larger stream, and, having greater 'hydraulic 

 mean depth,' it is at most points the quicker also. 

 The East River is delivering more water into New 



York harbor than it carries back again to the 

 sound. 



Although the Hudson and other rivers flow into 

 New York harbor, and slightly raise its level, the 

 conditions illustrated in our diagram are very 

 nearly realized in the seasons when the fresh- 

 water discharge is at a mininium. One of these 

 seasons is the autumn ; the other is mid -winter, 

 when the land- waters are ice-bound. At such 

 times, the greater velocity being westward, 

 and the greater depth of water being that of 

 the westerly flow, there is, as a net result, a 

 circulation of sea-water through the harbor from 

 the sound to the ocean. In mid- winter this 

 circulation, renewing the water before it can 

 get chilled, and lowering the freezing-point, 

 by mixing sea-water with the river discharge, 

 serves to keep the port open to commerce. One 

 may form some estimate of the value of the 

 three or four degrees difference of freezing-points 



between sea and river waters, when it is remem- 

 bered that in severe winters Halifax, Portland, 

 and Boston have not closed before Philadelphia or 

 even Baltimore, 



This circulation also aids in maintaining the 

 channels over the bar, which could not exist if 

 the ebb and flood were equal, i.e,, if there were 

 no ' net-gain' of the sands swept to and fro ; for 

 the bar is but a broken portion of the cordon 

 littoral of which Sandy Hook and Coney Island 

 are dry parts. Its channels are maintained by 

 a slight preponderance of the seaward flow, as 

 our observations distinctly show. 



Another and nearly related advantage of this 

 circulation is, that the heavier sea-water runs 

 low, and sweeps the bed of the harbor ; whereas, 

 were the harbor tideless, the river outflows would 

 be superficial on reaching the basins, and these 

 basins would in course of time fill up. This super- 

 ficial flow of fresh waters on reaching the sea is a 

 well-know phenomenon. The clear sea-water, 

 with the full density of the ocean, may be pumped 

 up from a few feet below the entirely fresh water 

 of the surface at the mouths of many rivers, nota- 

 bly those of great discharge. 



The fresh waters that enter from the Hudson 

 and other streams play an insignificant part in 

 the physics of the harbor ; but the circulation of 

 the sea by way of the East River, although small 

 in quantity, is the element which determines the 

 superiority of New York harbor over nearly all 

 the 'sand-barred inlets' of the world. It is this 

 circulation which keeps the port open in winter 

 and sweeps the sand from its threshold, 



Henby Mitchell. 



THE RECENT ERUPTION OF MAUN A LOA. 



Mauna Loa is again in eruption after an inter- 

 val of six years since the lava-stream of 1881, 

 which so closely grazed the town of Hilo, The 

 present flow is on the south-west side of the 

 mountain, entering the sea immediately north of 

 the flotv of 1868, or, rather, two miles from it. 

 The source is about twelve miles farther up the 

 mountain than in 1868, or twenty miles from the 

 sea. An aneroid observation gave 5,700 feet 

 elevation for the lower end of the fissure. The 

 points of emission ranged for three miles along 

 the vertical fissure, which appeared to extend 

 some two miles higher. 



On the evening of Jan, 16 a sharp jet of lava 

 was observed shooting up from the Mokuaweoweo 

 ' caldera ' at the summit of Mauna Loa to an 

 immense height, lasting ten minutes, with grad- 

 ual subsidence. This is the common precursor of 

 a flow from a lower point. The actual flow 



