350 The Lanibeth Ohservaiory . [July, 



and tlie other with a vertical set of micrometers. The foot screws 

 press against flat arms of metal, which are a fixed and integral part 

 of the stand, to obviate lateral ivriggh. 



The transit instrument, which will fix the longitude of the 

 principal stations, has an adjustment for the hearings of the pivots, 

 because the plan, now universally adopted in large instruments for 

 fixed observatories, of banishing entirely these adjustments, could 

 not be conveniently adopted in peripatetic instruments, that must 

 be moved from station to station. Each pivot is carried on a 

 triangle of iron resting on three screws, furnishing means of rough 

 adjustment for horizontahty. The connection of the pivot with 

 the triangle is, however, managed through a kind of ball and 

 socket intervention, in which the socket or cup is attached to the 

 triangle, while the broad convex spherical surface representing 

 the ball is the bottom of the metallic piece which supports the 

 cylindrical bearing of the pivot. One pivot allows of vertical, and 

 the other of horizontal adjustment ; in both cases the ball and socket 

 adapting itself with ease and exactness to the movement efiected. 

 Four levels for securing horizontahty, liang from S2)indles mounted 

 transversely upon the centre-piece of the telescope itself, and in 

 such a way as to remain in free and constant adjustment during 

 the rotation of the telescope upon its transverse axis. 



The chronographs, or time-registers, prepared to make the 

 transit observations to be recorded upon jjaper by electrical agency, 

 are chiefly remarkable for a most exquisite contrivance for securing 

 uniformity and exactness of movement, which is the invention of 

 M. Foucault, of the Imperial Observatory of Paris. A barrel 

 is carried round every two minutes by a tram of clock- wheels, and 

 two companion points so press upon paper stretched over the barrel, 

 and saturated wth an appropriate chemical agent, as to allow a 

 deep purple dot to be impressed on the j)aper every time the 

 pendulum of a clock sends a momentary current of electricity 

 through one point, which in practice is at the completion of each 

 second, while the finger of the observer does the same thing at will 

 with the other point, to register the time of the observation, so 

 interpolated between the seconds-dots. The movement of the 

 barrel is regulated to rate and uniformity — first, by a fly-wheel, 

 something like a miniature paddle-wheel, which revolves rai)idly 

 in a hollow case furnished with a peripheral slit, which can be 

 enlarged or diminished, to secure more or less aerial resistance, as 

 it may be desired. But in addition to this, the double balls of 

 the ordinary Watts' steam-governor are attached in such a way 

 that when they diverge from each other with too rapid motion, 

 they lift the weight of a lever-mounted counterpoise by rods 

 coming down from the middle of the diverging arms. The more 

 rapidly the balls revolve the more load they take upon themselves 



