THE SAROS AND THE TOTAL ECLIPSE OP 1806. 683 



rods further up, in the spring, were shells twelve mm. in length and six and one- 

 half to seven whirls, and these were probably the mature shells of the same 

 species. In other streams the shells were more globose, the suture more im- 

 pressed, of one whirl less and two mm. less in length. 



18. Of the Physas there seemed to be three varieties, though they would all, 

 perhaps, be considered as Physa Neterostrophe, Say. One of about the typical 

 shape is only nine mm. in length, with six whorls. The second is fourteen mm, 

 in length with seven whirls. The third, while as wide as the last, is two or three 

 mm. shorter, and nearly one whorl less. It has considerable resemblance to 

 Physa andllaria, Say. All three of these varieties are very fragile. 



There are doubtless several other water shells in addition to the Unios. I 

 found several species of the latter, but my opportunities were nofr sufficient to at- 

 tempt making a list of what may be found. 



The difference between the shells found here and at points only a hundred 

 miles away, is very great. I have found various species at Denison which are not 

 found here, and some that are plenty here do not extend to that part of the State. 

 The conchology of our country cannot be properly understood until specimens 

 and facts are gathered from every hundred miles, or even shorter distances. 

 When that is done, theories can be given and conclusions reached without so 

 much of guess-work or imagination as now. 



ASTRONOMY. 



THE SAROS AND THE TOTAL ECLIPS^;. OF 1806. 



BY WM, DAWSON, SPICELAND, IND. 



Having lately found much interest and practical value in the use of the 

 Saros, I thought perhaps other tyros of astronomy might also find entertainment 

 in some instruction on this point. Possibly some readers of the Review may 

 not know what I mean by Saros. It is the Chaldean Period of 18.03 years, after 

 which eclipses of the sun and moon recur in nearly the same order and kind as 

 they were before. That is, you take the time of an eclipse — say June 11, 188 1, 

 at midnight or i2h., and add 18.03 years, you get the time, very nearly, of 

 another eclipse of the same kind, and this addition may be carried on for ages in 

 the future without missing the time of recurrence of an eclipse more than a few 

 hours — half a day at most. And all eclipses that we ever have of both sun and 

 moon, recur or have occurred in the same way and order in this respect. This 

 period is believed to have been found out by the ancient Chaldeans and used by 

 them in predicting eclipses. True, it does not show the time of any individual 

 eclipse to a minute or second, as the adept mathematician obtains by long and 



