302 



KNOWLEDGE 



[Nov. 16, 1H33. 



mere fraction of the expense of the much more cumbrous 

 transit of Venus. Take, for instance, one of the little 

 planets which sometimes approach to within 70,000,000 

 miles from the earth. When the opposition is drawing 

 near, a skilled observer is to be placed on some suitable 

 station near the Equator. The instrument he is to use 

 should be the heliometer. It can be used to measure stars 

 at a greater range than is attainable with the filar micro- 

 meter. The measurements are to be made in the evening 

 as soon as the planet has risen high enough to enable it to 

 be seen distinctly. The observer and the observatory are 

 then to be transferred to the other side of the earth. How 

 is this to be done 1 Say, rather, how you could prevent it 

 being done. Is not the earth rotating around its axis, so 

 that in the course of a few hours the observatory on the 

 Equator is carried bodily round for thousands of miles 1 

 Before the morning dawns the observations are to be re- 

 peated. The planet is found to have changed its place 

 very considerably with regard to the stars. This is partly 

 due to its own motion, but it is also largely due to the 

 parallactic displacement arising from the rotation of the 

 earth, which may amount to as much as twenty seconds. 



The measures on a single night with the heliometer 

 should not have a mean error greater than one-fifth of a 

 second, and we might reasonably expect that observations 

 could be secured on about twenty-five nights during the 

 opposition. Four such groups would give the sun's dis- 

 tance to one-thousandth part. The chief difficulty of the 

 process arises from the motion of the planet during the 

 interval which divides the evening from the morning obser- 

 vations. This, it must be admitted, is a drawback to the 

 method. It can, however, be avoided by diligent and re 

 peated measurements of the place of the planet with respect 

 to the stars among which it passes. 



Let us hope that long before another transit of Venus 

 shall come round the problem will have been satisfactorily 

 solved by the minor planets. It will surely be expected 

 that I shall not close this lecture without an attempt to 

 say what the distance of the sun is so far as modern re- 

 search has gone. Reviewing all the diflerent methods, the 

 most probable value seems to be 92,700,000 miles. It does 

 not seem likely that this can be erroneous to the extent of 

 300,000 miles. The distance of the sun is one of the most 

 important constants in the universe. It is indeed a con- 

 stant in a very emphatic sense. The planetary perturba- 

 tions whicli affect so many other elements of the solar 

 system are powerless to touch this constant. Once the 

 distance of the sun has been measured, the telescope with 

 which the observations were made may moulder, and the 

 astronomer who used it may survive only in name, but the 

 work he has accomplished will remain true for countless 

 ages of the future. 



ON LUNAR DELINEATION. 



By the Rev. T. W. Webb. 



ATTENTIVE observation of the lunar surface has now 

 been, from various causes, comparatively dormant 

 on my part for a considerable time, though not, I hope, 

 without the prospect of renewal at some future day. But, 

 in the meanwhile, it has given me great pleasure to remark 

 with how much earnestness and success these inquiries have 

 been pushed forward by younger hands, and how far sele- 

 nography stands in advance of the position it occupied 

 during the earlier portion of the present century. There is 

 little occasion for surprise in this. During a period when 



a new and unprecedented interest has been awakened in 

 astronomical pursuits, and competent instruments — to say 

 the least of them — have been placed within the reach of aU 

 classes, nothing could be more natural than that attention 

 should be directed to an object so comparatively near, 

 so fi-equently visible in its returns, so accessible even 

 to small telescopes, and so crowded with strange con- 

 figurations as our own satellite ; and many an amateur, 

 who would be baffled from deficient optical power, or 

 want of experience, in an attack upon planetary details or 

 " close doubles," may be readily attracted by the broadly 

 developed surface of the moon, and become anxious to 

 know how he may turn to some useful account his oppor- 

 tunities of observation. And as far as the readers of 

 Knowledge are concerned, the recent interesting and 

 valuable series of papers by " F.R.A.S." would contribute 

 largely to encourage such a taste, and to lead young or 

 unpractised observers not merely to run over that strange 

 and fascinating scenery with a superficial gaze, but to study 

 its features more carefully in detail, and if they have some 

 facility with the pencil, to attempt its delineation. This 

 would certainly be a most interesting emjsloyment of spare 

 time, and with diligent perseverance might lead to a degree 

 of success which would fully justify the pains expended 

 upon it at the outset. It is with a view of assisting such 

 beginners that I propose to bring the following remarks 

 before them. 



We may begin by setting aside any discouragement from 

 the success that has attended photography — as to which it 

 may be sujiposed that from the marvellous development of 

 its resources it must soon supersede every other mode of 

 delineation. There is, indeed, much in such an objection, 

 but not enough to discourage the efforts of a careful 

 artist The great strength of photography lies in the 

 fact that it misses nothing. It would hardly be sup- 

 posed by those unacquainted with the subject how 

 fallible is the vision of an ordinary, sometimes 

 even of a trained eye, or how often features are over- 

 looked, especially in a hasty observation, which are 

 recorded by the camera. Nor can we question its great 

 advantage in point of rapidity, and, when suitably arranged, 

 facility of manipulation. But there is something to be said 

 on the other side. Photographic appliances are within the 

 reach of comparatively few of those who are qualified to 

 do good service, and the opportunities for really sharp 

 camera-work are limited, as compared with those for direct 

 eye-observation, for the evident reason that in the one case 

 a satisfactory result can only be obtained from such 

 uninterrupted steadiness of definition as may be long 

 waited for in vain, in the other the eye may be often 

 capable of laying hold on the true form and character of 

 an object, notwithstanding the interference of such atmo- 

 spheric agitation as would render the camera useless, or of 

 utilising the happy intervals in an unquiet hour, when a 

 momentary cleaiiag brings out in beautiful distinctness 

 features immediately lost again in impenetrable dift'usion. 

 For the general mapping of an extensive region, and laying 

 down its form and boundaries in due proportion, photo- 

 graphy is by far the best suited ; but there may always be 

 room for fiUing-in detail by the eye. It is best to under- 

 stand at once that if we hope to make our work really 

 useful, we must lay aside all ideas of representing large 

 areas — for one reason, because all that work has been 

 abundantly done already ; and for another, because, during 

 the time that would be necessarily occupied in such a 

 sketch, the illumination of objects lying respectively far to 

 the E. or W. would have altered sufficiently to make the 

 design inaccurate at one end or the other, however faithful 

 in separate details. And, for more than one reason, 



