September 8, 1892] 



NATURE 



445 



minutes over the whole of London between certain hours 

 when traffic is more or less suspended. If only tried for 



an hour each night some results might be obtained 



We have actually the mechanism for interplanetary com- 

 munication every night — why not use it ? " ^ 



Mr. Galton is careful to point out that his method of 

 signalling requires sunlight, and that the sfgnals are to 

 be flashed to Mars in the Earth's daytime ; the moment 

 of opposition therefore is at once out of the question. 

 Even with the Earth at either a or d in our Fig. 2, the 

 Sun and Mars would be 90° apart, and in any case the 

 signals would be visible to the Martians (if visible at all) 

 on the part of the earth lit up by the Sun. This does 

 not seem a favourable condition, or at all events the 

 most favourable one. 



Mr. Haweis' plan secures a much stronger contrast. 

 If it or something like it could be carried out, we can 

 imagine the inhabitants of Mars studying the delicate 

 earth crescent (with telescopes as powerful or more 

 powerful than our own bien entendu), whether as a morn- 

 ing or evening star, and then seeing rhythmic flashes, 

 reproducing the star included in the crescent of the 

 Ottoman flag well within the horns of the crescent. 

 Here we certainly get light on dark instead of light on 

 light. 



But there are other conditions of visibility besides those 

 we have so far discussed. Supposing the whole electric 

 energy of London turned on Mars would the volume of 

 light be sufficient to produce a valid signal ? 



It is worth while, quite independently of the popular 

 expectations of the present moment, to inquire into the 

 actual conditions of the problem, telescopes on Mars as 

 powerful as our own being always assumed. 



If we are armed with a powerful telescope, under the 

 best seeing conditions, first among which is its location 

 at a considerable elevation, we may perhaps reckon upon 

 using a power of 1000, that is, the object is magnified a 

 thousand diameters ; in other words, it is brought a 

 thousand times nearer. In the case of the moon, under 

 these conditions any part of her we might choose to study 

 could be examined, as if from London we were viewing 

 it over Yorkshire with the naked eye. 



The late Mr. Lassell, I believe, claimed as the highest 

 achievement possible with his 4-foot telescope in the 

 pure air of Malta, that if the lunarians were shaking a 

 carpet as large as Lincoln's Inn Fields he could see 

 whether it was round or square. This then would be 

 the ne plus ultra in the case of a body 240,000 miles 

 away. 



Now, if we take the nearest distance to Mars as 

 35,000,000 miles, as I have stated. 



Miles. 

 1,000,000 magnifying power would give 

 us the power of studying 



Mars as if it were 35 



100,000 ditto riitto 350 



10,000 ditto ditio 3500 



1000 ditto ditto 35.O00 ; 



We can put this differently. To the naked eye at 

 the distance of Mars i" = 160 miles. Were Mars 1000 

 times nearer 1" would become •16 mile. Now this at 

 first seems very hopeful, for the exterior satellite of 

 Mars has been seen in various telescopes. 



We have already learned that the power employed 

 last month at the Lick Observatory has not been so 

 much as 1000, but such that the planet has been 

 brought within a distance of 50,000 miles. Under these 

 conditions a line on Mars a quarter of a mile long will 

 subtend an angle of i", or two lines a quarter of a mile 

 apart should be separated and appear as doubles. 



The second satellite to which reference has been made 

 is only some 10 miles in diameter. We are justified 

 by the visibility of the satellite, then, in saymg that 



« Pall Mall Gazette, August i8. 

 NO. I 193, VOL. 46] 



away 



from 



us. 



if a space 10 miles in diameter could be lighted up, 

 as brilliantly as by sunlight, on the dark hemisphere of 

 the Earth when Mars is above the horizon and at peri- 

 helion, it could be seen from Mars by telescopes equal to 

 our own. 



London, of course, is more than 10 miles in diameter, 

 and we can imagine all the navies of the world with their 

 search lights to flash simultaneously towards the planet, 

 or to light up the clouds in a space as large as London, 

 but there then will remain the question of the intensity of 

 the light. What do electricians say is possible in this 

 direction ? 



Whatever the answer to this question may be, it seems 

 that signalling on Mr. Haweis' lines, light on dark, is a 

 more hopeful proceeding than that suggested by Mr. 

 Galton, and that on this system our conditions for read- 

 ing signals are far better than those on Mars, as our dark 

 hemisphere is much more exposed to our sister planet 

 than is hers to us. 



It is time now that we turn to those recent observations 

 of our neighbour which have given rise to the ideas 

 we have been discussing — ideas based upon the suppo- 

 sition that there is evidence which goes to show that 

 the Martians are signalling to us by digging " canals " 

 1000 miles long and 200 miles wide, and then 

 doubling them, and in addition lighting numerous signal 

 fires or flashing electric lights ! 



Here we approach a region of astronomical inquiry 

 which requires no enhancement of its interest by the 

 intrusion of popular delusions or imaginings, which, 

 moreover, for the next few months as details come to 

 hand, will have all eyes directed to it. 



It is not necessary to go further back than the year 

 1830 to appreciate the importance of the later inquiries. 

 In 1830 Beer and Madler made an admirable series of 

 drawings of the planet which enable d them to affirm the 

 existence of fixed markings, and having fixed markings, not 

 a long series of observations was necessary to determine 

 the period of the planet's rotation on its axis. 



In 1862 I (and many others) had no difficulty in recog- 

 nizing the features on the planet which Beer and Madler 

 had observed with smaller optical power thirty years 

 before. The instrument employed was a 6-inch Cooke 

 achromatic, which I still hold to be one of the finest 

 telescopes ever made. It enabled me to add details 

 to those before noted, and the observations left no 

 doubt on my mind that Mars had an atmosphere like our 

 own ; that its temperature did not vary many degrees 

 from our own ; that there were land surfaces and water 

 surfaces ; clouds and very obvious cloud drift ; polar 

 snows which melted with marvellous rapidity as the 

 perihelion sun made its full strength felt. Further, that 

 the changes in the appearances observed, especially in 

 the lighter or darker shading, depended upon clouds and 

 the smoothness or roughness of the water surfaces. 



This latter conclusion I arrived at from the fact that 

 the darkest markings, assuming them to be water surfaces, 

 were more or less land-locked, and that changes in some 

 of these surfaces were always most obvious close to the 

 land. It was clear also that the rapid melting of the polar 

 snow must be accompanied by tremendous inundations. 



I append, as an example of the kind of work done on 

 the planet with the small refractors generally available 

 thirty years ago, some extracts from a memoir I com- 

 municated to the Astronomical Society at that time.' The 

 large refractors employed added so far as I know very 

 little. 



'* Although the complete fixity of the main features of the 

 planet has been thus placed beyond all doubt, daily— nay, 

 >4t>«r/>— changes in the detail and in the tones of the different 

 parts of the planet, both light and dark, occur. These changes 

 are, I doubt not, caused by the transit of clouds over the different 

 I Menu R.-A.S. 1863, p. 17?. 



