THE ELECTRIC TELEGRAPH. 



"Now let us suppose, for example, that by the motion imparted 

 to the cylinder, an inch of the paper passes in each second under 

 the style. The style moved by the clock will therefore leave a 

 succession of marks upon the paper, at distances of an inch 

 asunder. But the particular distance of these marks is unim- 

 portant, nor is it material that the cylinder should be moved with 

 mathematical precision. If its motion for the short interval of a 

 second be practically uniform, that will suffice. 



When the object, a star for example, approaches the field of 

 view, the observer, with his eye to the telescope, holds his finger 

 over the key. He sees the star enter the field and approach the 

 first wire. The moment it crosses the wire, he presses down the 

 key, and the style gives a puncture to the paper on the cylinder. 

 In the same manner, when the star crosses the second and succeed- 

 ing wires, he again and again presses on the key, and thus leaves 

 as many distinct marks on the paper as there are wires. 



After the observation thus made has been concluded, the marks 

 on the paper are examined, and their distances from the pre- 

 ceding and following marks made by the pendulum style are 

 exactly measured, from which is inferred the fractional part of a 

 second, between the moment at which the star crossed each of the 

 wires, and the last beat of the pendulum. 



In this way the time of the transit is ascertained to the hun- 

 dredth part of a second. 



The Astronomer Royal, noticing this method of observing in 

 an address delivered before the Royal Astronomical Society, 

 said, that " In ordinary transit observations, the observer 

 listens to the beat of a clock while he views the heavenly bodies 

 passing across the wires of the telescope ; and he combines the 

 two senses of hearing and sight (usually by noticing the place of 

 the body at each beat of the clock) in such a manner as to be 

 enabled to compute mentally the fraction of the second when the 

 object passes each wire, and he then writes down the time in an 

 observing-book. In these new methods he has no clock near him, 

 or at least none to which he listens : he observes with his eye the 

 appulse of the object to the wire, and at that instant he touches an 

 index, or key, with his finger ; and this touch makes, by means 

 of a galvanic current, an impression upon some recording appara- 

 tus (perhaps at a great distance), by which the fact and the time 

 of the observation are registered. He writes nothing, except 

 perhaps the name of the object observed." 



He further observed that it was expected that by this method 



the irregularities of observation would be greatly diminished, 



whether because the sympathy between the eye and the 



finger is more lively than between the eye and the ear, remains to 



102 



