1869.] The Lambeth Olservatory. 347 



pass over the meridian. The angular intervals lying between the 

 culminating points of these standard stars are most accurately 

 determined by multiplied observations at established observatories ; 

 therefore all that is necessary, is to see that the graduations of the 

 circles indicate the precise angular intervals that they ought to 

 show for the stars that are employed in the test. There is, however, 

 one appendage to all vertical circles that is of a somewhat slip- 

 pery and suspicious character, but that is nevertheless at the bottom 

 of all correct indication, and must therefore be made the subject of 

 scrutiny. This is the level which is used to give the horizontal 

 point from which vertical elevation is reckoned. One portion of 

 the internal surface of the glass tube used in the construction of the 

 level is so ground that it just departs from a straight direction in 

 the line of length, so far as to give the air-bubble imprisoned on 

 the spirit an inclination to rest in the centre. But this surface 

 requires to be graduated so that the bubble travels over exactly the 

 same Hnear space on the graduation, for every equal angular change 

 in the elevation of one of the ends of the tube. To ascertain 

 whether the level is so made that this actually occurs, it is placed 

 uj)on a cradle of iron, and a delicate screw with a di\aded head is 

 turned hundredths of parts of a revolution at a time to raise or 

 depress one end of the cradle. If the level is well made, the bubble 

 travels one mark of the graduation for every hundredth part of a 

 turn of each thread of the screw. This appliance of test is so 

 dehcate, that if the finger and thumb are placed gently against 

 opposite sides of the bearer, the bubble moves from its place under 

 the expansion caused by the mere warmth of the living contact ; or 

 if, again, the hand be pressed firmly on the two-inch-thick slab of 

 slate that carries the instrument, the flexure of the seemingly 

 inflexible slate is immediately made apparent by the travelling of 

 the bubble. 



The optical performance of the telescopes of small surveying 

 instruments is tried by fixing a card, which has a series of lines 

 clearly traced upon it, some the fiftieth, some the seventy-fifth, and 

 some the hundredth of an inch asunder, 25 feet away ; telescopes 

 of difterent degrees of power should render the divisions of the one 

 or other of these series distinctly visible at this distance. The lines 

 which are the hundredth of an inch asunder subtend seven seconds 

 of angular measure at the distance of 25 feet ; that is to say, 

 a telescope which can show the closest lines as distinct visible 

 objects at the distance of 25 feet, will be capable of clearly dis- 

 cerning an actual object of even less than seven seconds of size. 

 The card containing the tracing of these lines also carries the exact 

 representation of the scale of the surveyor's levelling staff as it 

 would be seen at a distance of five chains and ten chains. These 

 scales are produced in most exquisite clearness and perfection by 



VOL. VI. 2 B 



