A VISIT TO AN OBSERVATORY. 109 



the field of view of the telescope we must have some 

 means of readily indicating the precise line down the 

 field which represents the meridian ; a thread or wire 

 has to be stretched across for this purpose. The magnify- 

 ing power of a large meridian circle is, however, so con- 

 siderable that any ordinary thread would look like a 

 coarse rope, and would be utterly unsuited for work 

 requiring nicety of observation. We must therefore 

 employ some fibre which shall be extremely fine, and 

 which shall yet be strong enough to admit of being 

 stretched into a perfect straight line, while a certain 

 degree of elasticity is also requisite in order to preserve 

 the straightness of the line with any permanence. These 

 various conditions are most completely complied with by 

 the beautiful thread of the spider. It is a somewhat 

 delicate task to stretch one of these exquisite filaments 

 properly over the little circular framework at the eye-end 

 of the telescope, but when it has once been done the web 

 of the spider is sufficiently durable to fulfil many years of 

 service. 



The method of observing the transit of a star will con- 

 sist of noting the time by the astronomical clock when it 

 passes behind the spider web. The due estimation of this 

 time taxes the skill of the practical astronomer; it de- 

 pends on the appreciation of the fractional part of a second 

 at which the transit takes place. Here, as in so many 

 other departments of science and of the arts, the aid of 

 electricity is invoked to give a subtleness to the work that 

 cannot otherwise be obtained. As the star passes behind 

 the spider web the astronomer taps a key which closes the 

 current, and impresses a mark on a revolving cylinder. 



