GEOGRAPHY AND ANTIQUITIES. 385 



on the day of new moon is 6.7 ; at the first quarter, 6.7 ; at full moon, 

 6.6 ; at the last quarter, 6.1. The first three numbers may be pro- 

 nounced identical with the average cloudiness of the entire period, as 

 already explained. The last might be suspected as indicating a gen- 

 eral law ; but if we compare the months for each year separately, we 

 shall find that they range from 5.0 to 8.0, showing that causes which 

 are independent of the moon's age exert a powerful influence upon 

 the degree of cloudiness. Moreover, we see that in 1841 and 1842, 

 the sky was remarkably clear at the last quarter, while the average of 

 the remaining year is 6.5, almost indentical with the results at the 

 other periods of the moon. 



This comparison therefore leads us to the same result as the former, 

 viz : that the Greenwich observations, which furnish the degree of 

 cloudiness of the sky every two hours, night and day, for some years, 

 give no countenance to the fact claimed by Sir John Herschel ; and 

 we have another example of the danger of drawing general conclusions 

 from observations loosely treasured up in memory, without testing 

 them by reference to recorded tables. 



ON THE TIDES OF THE WESTERN COAST OF THE UNITED STATES. 



At the Cleveland meeting of the American Association, Prof. A. D- 

 Bache, Superintendent of the U. S. Coast Survey, presented a com- 

 munication with the above title, of which the following is an abstract : 

 On the Atlantic, Gulf, and Pacific Coasts of the United States, there 

 are three quite different cases of the problem of the tides. On the 

 Atlantic, though the diurnal inequality is well marked both in interval 

 and height, much better so than on the Coast of Europe, it requires 

 large and carefully made observations to give the laws of the pheno- 

 mena. On the Gulf of Mexico, west of St. George's Island, the semi- 

 diurnal tides nearly disappear. On the Western Coast there are two 

 tides each day, but the diurnal inequality is very large. 



The study of the tidal phenomena then is very important to navi- 

 gation, for in San Francisco Bay, a rock which has three and a half 

 feet of water upon it at the morning high water, may be awash at high 

 water of the afternoon. Charts which are reduced in the ordinary way 

 to mean low water, will have no accurate significance, the soundings 

 being liable to an average error at either low water of the day of 1.18 ft. 



The observations now discussed were made by the officers of the 

 U. S. Coast Survey, in January, 1852, and January, 1853. They 

 were made hourly, except near the times of high and low water ; the 

 period of mean it was attempted to determine with precision. These 

 tides obviously present a case of large diurnal inequality in height, 

 which is at a maximum for both high and low water, when the moon's 

 declination is a maximum, and a minimum when it is zero. The in- 

 equality of interval amounts, when greatest, to two hours for high 

 water, and more than an hour for low water. 



The results for the half-monthly inequality, both time and height, 

 and of high and low water, are remarkably regular, considering the 



