116 THE TBANSIT OP VENUS AS OBSERVED AT EDEN. 



wire in its true position as a tangent to the sun's limb. Still, I 

 consider the above result to be very near the truth. I continued 

 to watch the planet for more than three minutes, and saw the 

 partial obscuration of the sun's limb by the planet's atmosphere 

 gradually diminishing until it disappeared altogether, when I left 

 the telescope at Oh. 24m. 48s. Mr. MacDonnell's estimate of the 

 same phenomenon was Oh. 25m. 14s. The discrepancy between 

 Mr. MacDonnell's results and my own shows how impossible it 

 is to fix the moment of a phenomenon of the kind, when the 

 motion is so slow and the change from darkness to light so gradual. 

 The slow rate of the planet's motion across the sun's disc may be 

 estimated by considering that it occupied over four hours in 

 describing so small an arc, not far exceeding one half of the sun's 

 diameter. The difficulty was still further increased by the planet's 

 path not being at right angles to the sun's limb, but inclined to it 

 at an angle of about 32 degrees. 



It may be asked why is the obscuration noticed on the outer 

 edge of the sun and not all round the planet. The answer is, 

 that rays from a much smaller portion of the sun are refracted 

 to the eye through the atmosphere near the point of contact 

 than through the other parts. 



As soon as we had concluded that ingress was complete, the 

 3-ineh diaphragm was substituted for the 2-inch and we proceeded 

 to take photographs, but iu doing so we were very much impeded, 

 and the quality of the pictures affected by the clouds which were 

 continually driving over the sun's face : indeed there were very 

 few minutes during which the sun was not more or less obscured. 

 Again the action of the wind on the yellow bag was so great that 

 the driving clock became almost useless, and I was obliged to 

 hold the telescope as best I could, with my eye at the finder, 

 whilst the plates were inserted and the flashing shutter released. 

 AYe made two attempts at a double image, but of course the 

 results were quite unreliable. On the whole we took about fifty 

 photogivaphs, very few of which I fear are of any value. At one 

 time we had to stop for twenty, and at another time for eighty 

 minutes, the sun being entirely obscured. On the whole the 

 expedition to Eden has not been a success, and I came away under 

 the impression that Eden, though a beautiful spot, and in many 

 respects a most desirable place to inhabit, is about the worst place 

 for astronomical observations that I ever visited. 



The Longitude of the Eden Observatory, as determined by 

 transits of stars at Sydney and Eden, recorded in the same 

 chronograph, is 9h. 59m. 36s. 25, and the Latitude by transits on 

 the prime vertical is 37° 3' 47". 



Sydney : Thomas Richards, Government Printer. — 1875. 



