Institution of Civil Engineers. 401 



degrees, the instrument being clamped in right ascension, by which 

 means it would be made evident whether the derangement of the 

 adjustment had any sensible effect upon the place of the planet. It 

 is the want of observations to accuse derangement which makes the 

 stars observed as moon-culminators less satisfactory than if they 

 were more widely spread in declination. 



With respect to observations of the moon, the author mentions 

 one set, originally suggested by Struve, but never carried into effect. 

 There are three observations which might be made when a bright 

 star is occulted by or reappears from under the moon's bright 

 limb : — 



1 . The time of disappearance or reappearance of the, star. 



2. Micrometrical measures of distance between the star and the 

 moon's bright limb, the clock-work and the wire micrometer with 

 the slipping piece being used. 



[This is the common observation of distance, and might be use- 

 fully applied to the case of a near approach.] 



3. Differences of right ascension between the moon and star, the 

 hour-circle being clamped as in ordinary transit observations. 



If the place of the moon be computed from these three observa- 

 tions, we ought to arrive at the same result ; and if we do not, the 

 difference between the first and second result arises from the moon's 

 irradiation, and will give a measure of it ; also a difference between 

 the second and third results would show some error in the mode of 

 taking the transit of the moon's limb, which is at present rather a 

 doubtful point in practical astronomy. If by certain corrections, 

 constant either to the observer or the telescope, these results can 

 be made to agree in each case, and always the longitude might be 

 determined in a shorter period, though with more calculation than 

 at present, and a greater certainty be obtained from transits of the 

 moon's limb. 



In conclusion, the author hopes that the attention of persons who 

 possess good equatoreals may be directed to the planets whenever 

 those bodies are favourably situated with respect to an observable 

 star. The adjustment is really nothing, and if pairs of stars above 

 and below be observed, any error arising from mal-adjustment can 

 be ascertained and allowed for. The artist will take care, if warned, 

 that the cross-axis shall be at right angles to the polar axis, and the 

 reductions, in ordinary cases, are very trifling, especially if by ju- 

 dicious grouping one reduction is made to serve for several observa- 

 tions. 



INSTITUTION OF CIVIL ENGINEERS. 



May 3, 1842. — " Description -of the Tunnels, situated between 

 Bristol arid Bath, on the Great Western Railway, with the methods 

 adopted for executing the works." By Charles Nixon, Assoc. Inst. 

 C.E. 



The works described in this paper comprised a large quantity of 

 heavy earth- work in tunnels, &c. ; they were commenced in the spring 

 of the year 1836, and terminated in the year 1840. The whole of 



