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the line crosses the meridian at an angle of 40° nearly, so that 

 I scarcely anticipated any sensible tremors. 



" I, however, have found that all the phenomena which I 

 observed at Dodder bank are reproduced here. 



" 1 . Disturbance of the mercurial horizon. The importance 

 of the observations made with this can scarcely be overrated. 

 In the case of stars, they give the index correction of a circle, 

 the verification of its divisions, and the means of eliminating 

 flexion ; nor are they less useful for the transit. But another 

 application of them, in which the reflected image of a tele- 

 scope's own wires is made to coincide with the direct one, is 

 perhaps even more valuable, and is coming into general use. 

 Both require almost absolute quiescence of the mercury ; an 

 agitation of it, which no other method of examination can ap- 

 preciate, will make the image nebulous and confused, or even 

 cause it to oscillate round its true place. Even a moderate 

 breeze of wind will do this ; and therefore, as the amount of 

 time during which such observations can be perfectly made is so 

 limited, the introduction of any extra disturbance is injurious to 

 an extent much more than proportioned to its actual duration. 

 Now, I find that with an outgoing train, the image is so much 

 disturbed as to make its coincidence doubtful for 4 m 36 s on an 

 average : in one instance even for Q m 1 7 s , at which time, as- 

 suming the velocity as given above, the distance must have 

 been 11,130 feet. The mercury vessel is the same which was 

 used in my former experiments ; but the magnifying power is 

 the habitual observing one of 250. The time of disturbance 

 for the incoming train is seven seconds less. During half this 

 time the image cannot be seen, and occasionally the waves on 

 the mercury are directly visible to the eye. 



"2. In bisecting a star, or making a coincidence of the 

 wire's images, the circle is moved by a fine screw. However 

 perfect its centre work may be, or however complete the adap- 

 tation of counterpoises and friction wheels, an amount of fric- 



