20S 



H. Creio — Period of Rotation of the Sun. 



c = value of one revolution of the micrometer 



screw in Angstrom's units. 

 V = velocity of light, in miles per second. 



A = displacement measured in micrometer revo- 

 lutions. 



A = wave length of line whose displacement is 

 measured. 



X = heliocentric latitude. 

 Where \ h = half the angle subtended at the center of the 

 sun's image by that portion of the slit cov- 

 ered by the sun's image. 



6 = inclination of the plane of the solar equator 

 to the ecliptic. 



cp = angular seaii-diameter of the sun as seen 

 from the earth. 



a = linear velocity of the earth in its orbit, in 

 miles per second. 



Observations. 



The following table includes all the observations made, ex- 

 cept three in which a radial slit was used. The difficulty of 

 setting on the end of such a line is so great that these have 

 been discarded, and only those made with a tangential slit are 

 retained. 



• These observations are arranged in the order of their solar 

 latitudes, given in column 7. 



Column 6 gives the difference between the readings of the 

 micrometer, on the eastern and western limbs of the sun re- 

 spectively. The relative equatorial linear velocities, computed 

 from the above formula, will be found in column 8. 



In column 9 is given the average discrepancy (in per cent) 

 among themselves of the ten settings which make up each 

 observation. May it not be that local currents — solar gusts — 

 have something to do with the large irregularities in, this 

 column ? 



Column 10 gives the quadrants in which lay the extremities 

 of the solar diameter under observation. This, together with 

 the latitude, completely determines the points at which the slit 

 was made tangent. 



Each weight in column 11 is the product of the cosine of 

 the latitude by a number depending on the definition and the 

 grating, and was determined from notes taken at the time of 

 observation. 



Six of the observations were made by Mr. Louis Bell, Fellow 

 in Physics of the Johns Hopkins University ; the others by 

 the writer. Neither of us, at the time, had the slightest idea 

 in what latitude we were observing. The value of the mean 

 relative equatorial velocity thus obtained is 



