204 



SCI£JJ^CU. 



[Vol. IX., No. 213 



The disturbed area embraces about 35,000 square 

 miles, and is elliptical in shape, the major axis 

 lying nearly east and west. It is limited to the 

 southward by the valley ^of the Ohio, and was but 

 slightly felt south of the river. The reported di- 

 rections of movement are, as usual, very incon- 

 sistent and of little value. It is generally reported 

 that two distinct shocks were felt, each of a few 

 seconds' duration, and with a small but very no- 

 ticeable interval between them. A low rumbling 

 was also generally observed as preceding and ac- 

 companying the shocks. 



With the exception of Professor Mendenhall's 



THE INDIANA EARTHQUAKE. 



observation, the times given are not accurate 

 enough to be of much utility. Coseismal lines, 

 therefore, cannot be obtained for this earthquake. 

 The speed with which a shock travels is so great, 

 and the area and distances, relatively speaking, 

 are so small, that it would require numerous time- 

 determinations of very great precision to warrant 

 any attempt to fix the coseismals. 



Mr. Everett Hayden has ' weighed ' the intensi- 

 ties, and has plotted, with his usual care and in- 

 telligence, the isoseismals herewith given. The 

 closed curves are neither symmetric nor co-axial, 

 and this seems to be certainly attributable, not to 



uncertainties of the reports alone, but to real 

 asymmetry in the distribution of the force of the 

 shocks, and to a real shifting of the axes of the 

 figures as the elastic waves of energy spread out. 

 It is not easy to make any comparison between 

 this earthquake and others which from time to 

 time occur in the valley of the Ohio, for it is the 

 only case since the New Madrid earthquakes of 

 1811-13 when definite data in sufficient quantity 

 have been gathered which would serve as the 

 basis of such an estimate. In a general way, it 

 may be said, however, that the intensity of the 

 disturbance in the central portions was, on the 

 whole, about equal to that exerted in the southern 

 portion of Ohio, central Tennessee, and Kentucky 

 by the Charleston earthquake of Aug. 31, 1886. 



CIRCULATION OF THE SEA THROUGH 

 NEW YORK HARBOR. 



Two derivations of the tide enter New York 

 harbor, one by way of Long Island Sound, the 

 other by way of Sandy Hook Bar. The one that 

 traverses the sound is much obstructed and 

 ' crowded,' so that it arrives upon the scene four 

 hours behind the other, and much augmented in 

 ' range.' 



These two tides meet, or pass into each other, at 

 Hell Gate, and give to the city portion of the East 

 River a composite ' rise-and-fall ' and a peculiarly 

 local system of tidal currents. The general scheme 

 of this meeting and composition is to be found in 

 the annual report of the coast survey for 1867, 

 much as I should give it to-day, so I will not 

 enter upon it here, but offer the accompanying 

 diagram as the types of the tidal profiles. 



The two figures are serpentine curves whose ele- 

 ments are those of the tides given in the tables of 

 published charts for the two entrances to New 

 York. From this diagram we observe that about 

 three lunar hours after the moon's transit, the sur- 

 face of the sound is at the same elevation as the 

 sea at Sandy Hook. Later, they differ, and more 

 and more widely, till at the sixth hour a maxi- 

 mum difference of height is reached, which ex- 

 ceeds five feet. Then a decline takes place, 

 till at the ninth hour the sound and Sandy Hook 

 Bay are again upon the same level. After this a 

 slope in the opposite sense develops, reaching a 

 maximum about the time of the next transit. The 

 first slope, that between three hours and nine, 

 is towards the sound, i.e., the sound continues 

 through this interval to be lower than the harbor. 

 The second slope is towards the harbor, and one 

 may remember it easily as that which reaches its 

 maximum at the ' southing of the moon,' and 

 creates the ' ebb-current,' so called. 



