378 Dr. N. Ernest Dorsey on the Surface -Tension of 



another horizontal and perpendicular to the screw, and the 

 third perpendicular to the plane of the other two. To do this 

 a mirror was mounted on the carriage perpendicular to the 

 length of the screw, and a telescope (placed about ten feet 

 from it) was focussed on the image in the mirror of a vertical 

 scale fixed to the telescope. If the carriage rotates about a 

 horizontal axis perpendicular to the screw, the image of the 

 scale will be displaced vertically with respect to the spider- 

 lines of the telescope. The carriage was moved several times 

 over the entire length of the ways, but no displacement of the 

 scale was noticed except what might be due to changing my 

 position with respect to the stand on which the telescope was 

 placed. 



The rotations about the other axes were tested in a similar 

 manner and with the same result. This proved that the 

 irregularities of the ways are at most very slight. 



To test the rigidity of the arms and mirror- clamps, the 

 carriage was screwed along until the spider-line in the tele- 

 scope on the carriage coincided with a well-defined crest. 

 It was then slipped back, leaving the nut in place on the 

 screw, and again moved up by hand to the nut. The line 

 again coincided with the crest. This pi'oved that moving the 

 carriage by hand produced no effect on the relative positions 

 of the carriage and its appurtenances. The carriage was then 

 again slid from the nut, and while held with one hand it was 

 struck several sharp, horizontal taps with a block of soft wood. 

 It was then moved back against the nut. If the telescope 

 and mirrors are clamped tightly the line again coincides with 

 the crest. This proves that slight jars of the screw will 

 introduce no error into the results if the telescope and mirrors 

 are well clamped. 



Having settled this point, the other adjustments, which 

 will now be described, were made ; and then the ways were 

 again tested by a method which will be described further on, 

 and which is much more delicate than the one first used. 



Leaving aside the accuracy of the screw and the motion of 

 the carriage, there are six fundamental adjustments to be 

 made : — 



1. The carbon filament and the axes of the lenses L A and L 2 

 must lie in a straight line. 



2. The beam of light must be composed of parallel light, 

 and 



3. It must be horizontal before reflexion by M x . 



4. The ways of the engine must be horizontal and at such 

 a height that the beam of parallel light may strike the mirror 

 M x ; and 



