1874.] W. M. Cam^heW—JVotes on the Transit of Venus. 248 



ence of the plate. The number of photographs taken will probably be 

 about 120 besides 6 of the Janssen plates. The ordinary wet process will 

 be used, the dry plates proposed to be used by other observing parties not 

 having been found to work satisfactorily. 



Captain W. J. Heaviside, R. E., of the G. T. Survey is expected to 

 joip the party for a few days, and he will also observe the contacts with a 

 fairly powerful telescope. 



Arrangements have been made for instantaneously recording on the 

 chronograph the time of exposure of each photograph and of each of the 

 60 pictures taken on a Janssen plate. 



All time observations will be recorded on the chroiiograph. 



This instrument consists of clock-work, driving four wheels, over each 

 of which a long tape of paper (such as used in telegraph offices) is passed and 

 drawn out by friction at a very uniform rate of half an inch per second. On 

 each tape the clock records seconds by means of a pricker worked electrical- 

 ly, while the observer to whom the tape belongs, has in his hand a tappet 

 key, by which he can work at will a second pricker alongside of the first. 



In order to trace the clock time on the paper, a mark is omitted at the 

 beginning of each minute. 



In addition to the apparatu^above described, there is a model of the 

 transit, similar to the one used at Greenwich. The advantage of this is, 

 that with a little practice, the observer becomes familiar with the phenome- 

 non and knows what to expect, or at least gets some notion of it, and fixes 

 his ideas of the exact phase he will seek to observe. 



By combined practice with two telescopes, the relative personal equa- 

 tions of the observers are arrived at, and such observations will be repeated 

 after the Transit is over. 



Familiarity with the model will also enable an observer to estimate the 

 occurrence of each phase very exactly, so that he can give a signal of warn- 

 ing to others, for instance in the present case, a signal will be given to Capt. 

 Waterhouse for the preparation and exposure of the "Janssen" plate, a 

 matter of great importance, as an error of a few seconds in exposing it would 

 render it useless, and Capt. Waterhouse will have no means of judging the 

 nearness of the contact for himself. 



The important phases for observation are : — 



1st. — The breaking of the ' black drop' at Ingress. 



2nd. — The forming of the same at Egress. 



We know that when well inside the sun's limb, Venus will appear to be 

 connected with it by a band of shadow, but we do not know exactly what 

 the behaviour of this band will be, whether it will burst at Ingress, or 

 form at Egress, in a well defined way, or whether it will fade out gradually 

 at Ingress and form slowly at Egress. 



