293 



Lieutenant Wilkes, of the United States navy, included between Sa- 

 brina Land and Repulse bay, and occupying an extent of about 400 

 miles. The Astronomer Royal is informed by General Sabine that 

 the 6th of December is rather early in the season for a visit to this 

 land, but probably not too early, more especially as firm ice will be 

 quite as good for these observations as dry land. It must, however, 

 be borne in mind that it is indispensable to secure observations both 

 of ingress and of egress in this tract, without which all the advan- 

 tages of the North American observations will be useless. For this 

 purpose it appears absolutely necessary to establish a chain of ob- 

 serving posts, and to furnish some means of comparing the clocks. 

 We are in possession now of two powers, unknown in former times, 

 applicable to this purpose. One is the galvanic telegraph, which 

 possibly (but not very probably) might be laid down in a temporary 

 way. The other is the use of steamers, by which the observers 

 would be distributed to their several posts, and which would be con- 

 stantly employed for some days before and some days after the transit 

 in running up and down the line of coast with a number of chronome- 

 ters, and comparing them with the stationary chronometers at each 

 observing post. It would be extremely desirable that the country 

 should be reconnoitered some years before the transit, in order to 

 ascertain at a sufficiently early time the practicability of these or some 

 equivalent plans, without which the risk of entire failure would be 

 great. 



Fourth. On the application of the method of the differences of ab- 

 solute times to the transit of 1882. 



For the ingress, the islands of Bourbon, Mauritius, and Kerguelen's 

 island, are very favorably situated for accelerating the ingress; and 

 the United States of North America for retarding it. For the egress: 

 Van Dieman's Land, Eastern Australia, New Zealand, and New Cale- 

 donia, will have the egress much retarded; while the United States, 

 the West India islands, and the coast of South America as far as the 

 Rio Plata, will have it accelerated. 



In the transits of 1761 and 1769 great difficulty was found to attach 

 to the observations of the internal contact of the limb of Venus with 

 the Sun's limb, from the phenomenon which in late years has attracted 

 attention under the name of Baily's Beads. The Astronomer Royal 

 expressed his opinion as entirely coinciding with that of Professor 

 Powell, that this phenomenon is simply due to irradiation, as arising 

 partly from dillraction, partly from fault of the telescope, and partly 

 from the nervous excitement of the eye. From his own experience 

 in two total eclipses of the Sun, in which he had taken great pains to 

 see the phenomenon, and had (as he believes simply because he took 

 care to see the Sun very distinctly) been unable to see the slightest 

 trace of it, he had not the smallest doubt that when proper care is 

 taken for distinct vision, the phenomenon will not be seen at all. He 

 referred specially to his delightful view of the very beautiful phenome- 

 non of the disappearance of the last portion of the Sun in the valleys be- 

 tween the lunar mountains, in the eclipse of 1851, which with less 

 distinct vision would probably have created strings and beads. This 



