Royal Astronomical Society. 145 



causes of the foundation of an observatory at Cambridge, we form 

 no trivial wish when we hope that the consequences of the future 

 may equal in importance those of the past. 



It is probably known to most of the Fellows, that the government, 

 on the representation of the Astronomer Royal to his Board of Visi- 

 tors, has sanctioned the erection of an altitude and azimuth instru- 

 ment for extra-meridional observations of the moon. For more than 

 a century all fundamental observations of stars and planets have been 

 made in the meridian ; and if sufficient frequency of observation could 

 be ensured, there is no reason to su])pose that any other species of 

 observation could be required. In the case of the moon, however, 

 there is a necessary loss of the meridian observation during a part of 

 the month, and occasional occurrence of clouds at the time of other- 

 wise visible transit ; to which it must be added, that theoretical 

 knowledge of our satellite is so advanced as to make it difficult to 

 carry it further by such sets of observations as can be procured with 

 meridional instruments. The Astronomer Royal has therefore de- 

 termined to commence the task of following the moon through her 

 daily course to an extent which will supply, or more than supply, 

 the failure of meridian opportunities ; and your Council feels a san- 

 guine presentiment that this abandonment of the meridian will be an 

 epoch in the history of astronomy. Tlie altitude and azimuth in- 

 strument is constructed on the same general principles as the ord- 

 nance zenith sector described by Mr. Airy at the meeting of May 

 13, 1842. It consists of very few parts, and these cast, and there- 

 fore rather massive ; and so important has it been considered to 

 unite the small parts, that the microscopes are cast in the same piece 

 with the rest, and are bored afterwards. The instrument turns on 

 pivots above and below. The revolving azimuthal frame consists 

 only of four casts — namely, the lower end, with a pivot and four 

 microscopes; the two sides, of which one has four microscopes ; and 

 the top, with its pivot. Tlie moving vertical circle consists of two 

 parts only ; one having one pivot, the graduated circle, and the two 

 ends of the telescope, the other having the other pivot. The extreme 

 firmness which is required in the construction of a theodolite (as it 

 may very properly be called), which shall give results comparable to 

 those of the fixed instruments, is of course accompanied by consider- 

 able loss of manual adjustment, and consequent increase of arithme- 

 tical reduction. Those who have any idea of the very serious labour 

 involved in the contemplated class of observations, as compared with 

 that required in meridional work, will feel that the present As- 

 tronomer Royal has dictated to his successors the motto by which 

 Ptolemy described Hipparchus, (piXoTrovos nal (piXuXijdris ; and they 

 will also feel that he has adopted it for his own. 



Your Council cannot but feel it to be a matter of congratulation, 

 that the principal public observers of Great Britain have, to no small 

 extent, begun to bear in mind that different observatories, situated 

 at no great distance from each other, should aim at diversified plans 

 of action and independent objects of investigation. While all have 

 recorded the places of the bodies of the solar system, and have lent 



P/iil. Mag. S. 3. Vol. 27. No. 1 78. August 1845. L 



