PHILOSOPHICAL SOCIETY OS WASHINGTON. 105 



Mr. Shellabarger, Mr. G. F. Talbot, and Mr. Curtis de- 

 scribed waterspouts or cloudbursts which they had witnessed 

 among mountains or hills, where the fall of water appeared to be 

 sudden and great. 



Mr, Henry remarked on the necessity of recorded statements 

 of facts giving fully the attending circumstances, and referred 

 to the investigations by Espy many years ago, and to a valuable 

 report on the subject of tornadoes in one of the Reports of the 

 U. S. Signal Service. 



Mr. J. H. C. Coffin made the following remarks on 

 Sumner's method in navigation. 



To find the longitude of a ship at sea by a chronometer, the 

 chronometer time of one, or more, altitudes of the sun or other 

 celestial body is noted, and reduced to mean time at Greenwich, 

 or other prime meridian, by applying the chronometer correction, 

 supposed to be known more or less accurately. Navigators 

 usually stop here with this part of the process. It is better to 

 go farther and find the hour angle at the prime meridian of the 

 body observed. This in the case of the sun is the apparent 

 time, reckoned in A. M. from the lower meridian. 



The local hour angle is found from the latitude of the place, 

 tlie declination, or polar distance, of the body, and its corrected 

 altitude. The difference of the local hour angle from that at the 

 prime meridian is the longitude in time. 



The declinations of bodies observed at sea are known far more 

 accurately than is requisite for navigation. The altitude is always 

 uncertain, 2' or more even when very carefully observed, owing* 

 to the variable refraction of the sea horizon. The latitude is the 

 most uncertain element, as it is brought forward or carried back 

 by the dead reckoning, from some determination made at a time 

 several hours distant. 



Here, then, is a simple case of finding the locus of an equation 

 with two unknown quantities ; and we may assume values of the 

 latitude within reasonable limits and compute the corresponding 

 longitudes. Unless the altitude is very great, or the latitude 

 very uncertain, it is sufficient to assume two latitudes and find the 

 corresponding longitudes, i. e. determine two points of this line 

 of position. 



