II 4 RAMBLES AND REVERIES. 



the astronomer had something to do with the 

 discovery of gravitation. This great law, like 

 many other wonderful things, seems not to have 

 been discovered solely by the labours of one man, 

 but it was certainly Newton, not Flamsteed nor 

 Descartes, who first gave it scientific form, and 

 gave such mathematical demonstration of it as has 

 placed it for ever beyond dispute. 



The first instrument seen was the great transit 

 telescope, which by means of its wonderful spider 

 lines enables the observer to determine the instant 

 at which celestial bodies cross the meridian. The 

 telescope is so mounted that its optical axis may 

 move only in the plane of the meridian, and the dome 

 above it can be opened to the sky in the same plane. 

 So delicate are the movements of this apparatus 

 that a change of a few feet in the adjustment of 

 the mercury vessel will, to the confusion of John 

 Hampden and all other flat-earth theorists, indicate 

 the rotundity of the earth within that short space. 

 Connected with the transit instrument, but in 

 another room, is the chronograph, which registers 

 on a revolving barrel covered with prepared paper 

 the manipulations of the observer at the telescope. 

 Here a very curious fact was pointed out. Different 

 observers see and transmit phenomena in different 

 periods of time. As a psychologist would say, 

 cerebration varies in different individuals. By a 

 subtle arrangement these variations of mental 

 activity can be measured, and when the amount 

 of variation, or " personal equation," is known for 

 a certain person, his observations can be compared 



