404 Mr. David Gill [May 29, 



After four or five hours of this work the observers have had 

 enough of it ; they have, perhaps, observed fifty or sixty stars, they 

 determine certain instrumental errors, and betake themselves to bed, 

 tired, but (if they are of the right stuff) happy and contented men. 

 At the Cape we employ two observers, one to read the circle and one 

 to record the transit. Four observers are employed, and they are 

 thus on duty each alternate night. Such is the work that an outsider 

 would see were he to enter a working meridian observatory at night, 

 but he would find out if he came next morning that the work was by 

 no means over. By far the largest part has yet to follow. An obser- 

 vation that requires only two or three minutes to make at night, 

 requires at least half an hour for its reduction by day. Each obser- 

 vation is aflected by a number of errors, and these have to be deter- 

 mined and allowed for. Although solidly founded on massive i)ier8 

 resting on the solid rock, the constancy of the instrument's position 

 cannot be relied upon. It goes through small periodic changes in 

 Level in Collimation and in Azimuth, which have to be determined 

 by proper means, and the corresponding corrections have to be com- 

 puted and applied ; and also there are other corrections for refrac- 

 tion, &c., which involve computation and have to be applied. But 

 these matters would fall more properly under the head of a special 

 lecture upon the transit instrument. I mention them now merely 

 to explain why so great a part of an astronomer's work comes in the 

 daytime, and to dispel the notion that his work belongs only to the 

 night. 



One might very well occupy a special lecture in au account of the 

 peculiarities of what is called personal equation — that is to say, the 

 difiereut time which elapses for difierent observers between the time 

 when the observer believes the star to be upon the wire and the time 

 when the finger responds to the message which the eye has conveyed 

 to the brain. Some observers always press the key too soon, some 

 always too late. Some years ago I discovered, from observations to 

 which I will subsequently refer, that all observers press the chrono- 

 graph key either too soon for bright stars or too late for faint ones. 



Other errors may, and I am sure do, arise both at (Treenwich and 

 the Cape from the impossibility of securing uniformity of outside and 

 inside temperature in a building of strong masonry. The ideal 

 observatory should be solid as possible as to its foundations, but 

 light as possible as to its roof and walls — say, a light framework of 

 iron covered with canvas. But it would be undesirable to cover a 

 valuable and permanent instrument in this way. 



But here is a form of observatory which realises all that is 

 required, and which is eminently suited for permanent use. The 

 walls arc of sheet iron, which readily acquire the temperature of the 

 outer air. The iron walls are protected from direct sunshine by 

 wooden louvres, and small doors in the iron walls admit a free circu- 

 lation of air. The revolving roof is a light framework of iron covered 

 with well-painted papier mache. 



