xhi REPORT— 1843, 
of the machine. As the first type wheel shifts every two seconds, and as the 
plunger may leave the mercury at any instant of time, the hammer might 
strike during the shifting of the type wheel and produce a blurred or imper- 
fect impression ; to obviate this a coutrivance is introduced, for the purpose 
of continuing the current for an instant after the plunger leaves the mercury, 
whenever the contact is broken at the moment the type wheel shifts. By 
means of this addition all the observations are registered with regularity and 
distinctness. 
The accuracy of the recorded observations is not in the slightest degree 
influenced by the rate of going of the clock. Whether the rate be accelerated 
or retarded, the same number is always printed for the same degree of ele- 
vation of the mercury. The only circumstance affected by the variation of 
the time of the clock is the time of the observation. 
The elevation of the mercury in the tube by the insertion of the plunger 
gives rise to no error, because the observation is recorded only at the moment 
the plunger leaves the mercury, and when the mercury is consequently at its 
proper level. 
A description will also be presented to the Association by Professor 
Wheatstone of an Electrical Apparatus which has been established in the 
cupola of the Observatory; the cost of this apparatus has been defrayed by 
private subscription. 

