682 Prof's. Morley and Miller : Report of an Experiment 



in sodium light, we found the central part of a series of some 

 700 interferences which are brighter than the adjoining 300. 

 With no long search, we could see interferences in white light, 

 although we had provided no screw for moving a mirror with 

 its surface always parallel to a given surface. This we had 

 avoided, in order to have everything about the two arms as 

 symmetrical as possible. 



We now computed the direction and velocity of the motion 

 of the centre of the apparatus by compounding the annual 

 motion in the orbit of the earth with the motion of the solar 

 system towards a certain point in the heavens. During part 

 of August, the whole of September, and nearly all of October, 

 this motion never coincides with the plane of our apparatus. 

 For other dates, there are two hours in each day when the 

 motion is in the desired plane, except for two days when 

 the two hours coalesce into one. At the beginning of June, 

 the two hours are about ll h 20 ra a.m., mean solar time, and 

 9 h 50 m p.m. At the time of our last set of observations, 

 July 5th to July 9th, the hours were ll h 40 m a.m. and 

 8 h 20 m p.m., local mean time. 



After many trials, with filar micrometer, and with a 

 scale on mirror 8, we found it advisable to accumulate a great 

 number of observations made as rapidly as might be. What 

 we had to do, in presence of all the local disturbances of 

 density of the air which sometimes made observation im- 

 possible and always made it difficult, was as if we were 

 trying to measure the diurnal solar atmospheric tide. If 

 Ave could vary the period of this tide at will by controlling 

 the revolutions of the earth, we should doubtless get a result 

 sooner by accelerating the latter and making a great number 

 of observations in a given time, rather than by retarding the 

 period in order to measure with very great precision the 

 hourly height of a barometer. We therefore proceeded as 

 follows: — One observer walked around with the moving- 

 apparatus, his eye at the telescope, while he maintained the 

 rotation by an occasional gentle pull on a cord so fixed as not 

 to bring any strain to bear on the cross arms of the apparatus. 

 The room was darkened. The other observer also went around 

 with the apparatus; as an index showed the azimuth of the 

 apparatus to be that indicated by one of 16 equidistant marks, 

 he called out the number or some other signal. The first 

 observer replied with the reading for the given azimuth, 

 which the second observer recorded. The next azimuth was 

 called at the proper instant, the reading given, and so on. 

 Half the time, perhaps, the observations were interrupted before 

 they became numerous enough to be useful, being stopped 



