May 4, 1883.] 



• KNOWLEDGE 



265 



9-28 times that of the ordinary lamps. The cost of lighting 

 the thoroughfare appears to be, by the ordinary system, 

 ;£99. Ts. 4d. per annum for twenty-two lamps, and by the 

 cost system, £379. Gs. Id. per annum for a like number of 

 lamps. The general result appears to be, therefore, that 

 the lighting with the Siemens burner gives nine times the 

 luminosity, at about three and three-quarter times the 

 cost of the superseded lamps. None, however, but those 

 blind to the beauties and uses of the electric light will 

 be prepared to admit of the Siemens lamp that it 

 "enables shades of colour to be accurately detected" — 

 or that this is an advantage which (as is claimed in 

 the pamphlet issued by the Siemens Company) "cannot 

 be accomplished satisfactorily by any other artificial 

 light Nor is it altogether correct to designate gas- 

 light as the "light of lights." There is no doulit that, 

 however young and inexperienced the electric light in- 

 dustry may be, we should never have seen the recently- 

 effected improvements in gas-lighting had it not been for 

 the fear of a close and determined competition on the part 

 of electricity. 



Another very interesting exhibit is Clark's " Recupera- 

 tive Shadowless Lamp," in which the llarae is in the form of 

 a ring, the whole of the light being projected downwards, 

 instead of mainly upwards, as in the Siemens. Want of 

 space, however, will not permit of anything more than a 

 reference to this lamp. 



The Exhibition will have closed Ijy the time this appears, 

 and many of the exhibitors will have packed up and 

 cleared away. If there is any lesson to be drawn from the 

 exhibition, it is that gaslight is not by any means a thing 

 of thj past, nor is it necessary that our halls and rooms 

 should lie rendered untenable by the products of combustion, 

 as, in the majority of the lamps exhibited, these products 

 are entirely carried away, and help, inductively, to ventilate 

 the room. So far as electric lighting is concerned, it seems 

 impossible to say much concerning its prospects till the 

 Board of Trade have finished their share of the work in 

 granting "orders." This much, however, may be said, the 

 companies will be a long time before they recover from the 

 blow they received last summer at the hands of stock- 

 jobbers. 



THE MOON IN A THREE-INCH 

 TELESCOPE. 



By .1 Fellow of the Royal Astronomical Society. 



IN commencing our examination of some of the typical 

 and more remarkable objects on the moon's surface, 

 we will suppose that she is between three and four days 

 old. Arming, then, our three-inch telescope with a power 

 of 120, we proceed to direct it to her face ; and a very re- 

 markable spectacle it is which will present itself to the 

 student making his maiden observational essay on our 

 satellite. He will first be struck by the number of ring 

 plains, of which we have briefly spoken in our introduc- 

 tory paper, between the circular bright western limb of 

 the moon, and the " Terminator " ; by which name, as we 

 have previously explained, the boundary of light and dark- 

 ness is known. The Sea of Conflicts (A in our map on 

 p. 22.'3), and part of the Sea of Fertility (X) wOl also 

 strike his eye. Before proceeding, however, to scrutinise 

 the various olijects contained within the liright crescent of 

 the moon, it will be interesting to shift her image in the 

 field of view of the telescope. If this be done, it will be 

 found that the whole of the moon is visible ; the dark 

 limb looking like a very ghost on the black background 



of the sky. Moreover, if the atmospheric conditions 

 are favourable, a certain amount of detail will be 

 readily seen upon this dark portion of the moon, 

 a bright spot, Aristarchus (148 in our map), and a 

 dark one, Grimaldi (272), being the most conspicuous 

 objects, ^^'e may shortly say here, with reference to this 

 phenomenon, that it is the effect of earth-shine. Five 

 minutes' study of the diagram illustrating the changes of 

 the moon which appears in every elementary work on 

 astronomy that has ever Vicen written, will show that when 

 the moon is new to the earth, the earth is full to the 

 moon. Moreover, we present a disc to our satellite more than 

 thirteen times the size of that which she exhibits to us, and 

 hence it will be seen that the amount of light we send her 

 at the time of her inferior conjunction (or when she is 

 " new ") must be very considerable. Of course, as the moon 

 waxes to us, we wane to her, so that it is only during the 

 first and last few days of every lunation that this earth- 

 shine renders the dark side of the moon visible. Having 

 satisfied ourselves as to its visibility, we will return to 

 the illuminated crescent Xow, the craters and plains 

 visible at the time of which we are speaking all present, 

 more or less, an elliptical outline, the ellipticity becoming 

 more and more marked as we approach the bright limb. If 

 the student will regard a terrestrial globe from a little 

 distance, he will at once understand that this is an eft'ect of 

 perspective. In fact, while the Sea of Conflicts (A) seems 

 to have its major axis North and South, it in reality 

 lies from East to AVest ; this great, dark plain, measur- 

 ing only 2S1 miles from North to South and 355 miles 

 from East to West ! We have called it a plain, but 

 a little attention will show undulating ridges on parts 

 of its surface. Its greenish grey tint will be noted, 

 too. To the East of this " sea," Picard (4) will be seen. 

 To the West of this is a white spot, which is a rather 

 mysterious object, having been seen to present the most 

 varying appearances. North-east of Condorcet (5) is the 

 Promontorium Agarum, a kind of peninsula projecting 

 into the Mare. This is a striking object when the moon is 

 nearly sixteen days old. Cleomedes (12) is a fine forma- 

 tion, about seventy-eight miles in diameter. It has a trifid 

 mountain in its interior. On its eastern w^all is situated a 

 very deep crater plain, Tralles ( 13 in the map). There is 

 a central mountain on the floor of this, too. Endymion (27) 

 is a circular plain (elliptical as it appears in the telescope). 

 Its western wall rises in places to a height of upwards of 

 15,000 feet Directing our instrument now towards the 



Petavius. Jloun s Afe. :J24 days. 



southern half of the lunar crescent, we arrive at Lan- 

 grenus (338) and Vendelinus (339), the former a splendid 

 object, with a bright central hill. And now we come to 

 that grand object, Petavius, which, as illustrating several 

 typical lunar features, we have here drawn. 



As seen in a three-inch telescope with a power of 120, the 

 moon's age being 3 24 days, it will be noted how the wall 

 is divided by narrow valleys. The mountain in the convex 



