Williamson.] boJi [Sept. IT, 1875. 



METEOROLOGICAL OBSERVATIONS TAKEN ON THE NILE 

 BETWEEN CAIRO AND THE FIRST CATARACT, DURING 

 JANUARY AND FEBRUARY, 1873. 



By Lieut. -Col. R. S. Williamson, 



UNITED STATES COEPS OF ENGINEBES. 



{Read before the American PMlosopMcal Society, September 17, 1875.) 



San Francisco, Cal., July 26, 1875. 

 To the Secretary of the American Philosophical Society : 

 Dear Sir : 



I send you two sheets of Meteorological Observations which I made 

 during January and February, 1873, on the Nile, thinking that they 

 might be considered of sufficient interest to find a place among the i>rinted 

 Proceedings of the American Philosoijhical Society. 



Wliile tlie general character of the climate of that country is wtll 

 Jinown, I have not heard of there having been pviblished any regular 

 series of observations of the wet bulb from there ; and the large number 

 of tourists who annually visit that river, the majority of whom are 

 Amei'icans, makes facts concerning it of more than usual interest. 



I had had made in Cairo, before starting uj) the rivei', a box two feet 

 square, four sides of which were of lattice bliuds, so that the instruments, 

 when suspended in it, were perfectly protected from the direct rays of the 

 sun, while the wind i:)assed freely through it. One side of the box was pro- 

 vided with double doors, one or both of which could be opened or closed 

 at pleasure. The box was placed on a table on the upper deck of the 

 boat, and securely fastened to it. The bulbs were about ten feet from 

 the water. Usually there was an awniag above. From frequent experi- 

 ments I faund that there was no difference between the readings of the 

 instruments when the doors of the box were open or closed. 



The principal instruments were two sensitive identical Thermometers, 

 which read alike when the bulbs were dry. They were made by James 

 Green, of New York, and were of the best construction. There was also 

 a minimum Thermometer, but not of so nice a construction. 



The reductions were made by means of the tables in Profession Papers 

 of the Corps of Engineers, No. 15, a copy of which is in the library of the 

 Society. 



The boat went up the river as far as Assouan, at the foot of the first 

 Cataract, and six degrees of latitude south from Cairo, and returned. 



Yours very truly, 



R. S. WILLIAMSON, 

 Lieut.-Col. United States Engineers. 



