Nov. 16, 1883.] 



♦ KNOV/LEDGE 



301 



first finger of my right hand on the handle of the brake. 

 Even using one finger to check the machine in this manner, 

 I have frequently lifted the hind wheel when I have been 

 compelled to apply the brake rather suddenly, and that 

 when I have been riding up hill. 



A correspondent to whom I am indebted for a thought- 

 ful communication, asks me if the "Humber" " would not 

 be improved on the score of safety by throwing more 

 weight on the hind wheel, so as to keep it more firmly on. 

 the ground f " 



My reply, given after a year's continuous practice in 

 riding the machine, is that, paradoxical as it may appear, 

 the more weight you place upon the hind wheel the more 

 dilficult it becomes to control the machine. 



The secret of riding the " Humber " is to lean well 

 forward so as to take as much weight oft' the hind wheel as 

 pos.sible without going over the handles. When going 

 down hill, if the hind wheel gets into a rut or deep dust or 

 stifl' mud and the machine will not answer in steering, the 

 only way to get over the difficulty is to lean forward as far 

 as you safely can, to apply the brake cmUiousli/, and to 

 drive forward against the brake. This will steady th3 

 machine directly, and cause it to answer the steering 

 perfectly. 



THE SUN'S DISTANCE. 



By Professor R. S. Ball, LL.D., 



ASTBONOMER-EOYAL FOR IRELAND. 



(Continued from page 285.) 



IT was on the first night of the present century, a date 

 very easy to remember, that the first of the minor 

 planets was discovered ; a few others were quickly added, 

 and forty years later the career of discovery was re-opened. 

 Fresh planets were added by units, by tens, and by scores, 

 until at the present moment we recognise about 240 

 of these objects revolving around the sun in the gap 

 between Mars and Jupiter. 



The sun, which controls the mighty planets of our 

 system, does not disdain to guide with equal care the tiny 

 globes which form the minor planets. Each revolves in 

 an elliptic orbit, and at certain times some of them ap- 

 proach near enough to the earth to have their distances 

 measured. The observations can be made with very great 

 precision ; they can be multiplied to any extent that may 

 be desired. They can be discussed with geometrical ac- 

 curacy, so that from them one result and only one can be 

 deduced. 



In speaking of these planets as small objects, I must be 

 understood to use the words in a comparative sense. They 

 are small as planets, but perhaps the area of an average 

 one might be comparable with that of a good-sized English 

 county. The discoveries of minor planets have added enor- 

 mously to the labours of those astronomers who compute 

 the movements of the solar system. Tlie task of predicting 

 the places of the minor planets has been for many years 

 most ably accomplished in the Berlin " Ephemeris." The 

 editor must bo sometimes alarmed at the daily extension of 

 his list. 



Many of these bodies are of but little interest. Perhaps 

 it would be no great harm to allow a good many of them 

 to get lost again and lie no more followed by telescopes or 

 computers. Some of these little bodies have, however, a 

 great astronomical future. They seem destined to tell us 

 more faithfully than Venus or than Mars what is really the 

 distance from the earth to the sun. The very small ones 



will not answer ; they can only be seen in very powerful 

 telescopes, and they do not admit of being measured with 

 the necessary accuracy. It is also obvious that the planets 

 to be chosen must be as near to the earth as possible. 

 They should have orbits possessing a high degree of eccen- 

 tricity, so that w-hen in perihelion and in opposition simul- 

 taneously the approach may be made as close as possible. 

 Under favourable circumstances a minor planet will 

 approach the earth to a distance but little more than three- 

 quarters of the distance of the :,un. These various condi- 

 tions limit the number of minor planets available for this 

 purpose to about a dozen, of which one or two will usually 

 be suitable each year. 



About two years ago Mr. Gill invited astronomers in 

 the Northern Hemisphere to co-operate with him in the 

 Southern in an attempt to apply this method to Victoria 

 and to Sappho. It became my duty to co operate in this 

 scheme so far as my opportunities went. As the planet 

 pursues its course through the sky, and as the sky is every- 

 where studded over with countless myriads of minute star.s, 

 it is evident that the planet itself so like a star will always 

 have some stars in its immediate neighbourhood. As the 

 movements of the planet are well known, it is possible to 

 foretell where it will be on each night that it is to be 

 ob.^erved. 



The scheme was completely drawn up many months 

 before the observations were to commence. Each observer 

 who participated in the work was thus advised beforehand 

 of the stars which were to be employed each night. 

 Viewed from any part of the earth, from the Cape of 

 Good Hope or from Great Britain, the positions of the 

 stars are absolutely unchanged. Their distance is so 

 stupendous that a change of place on the earth displaces 

 them to no appreciable extent. But the case is ditierent 

 with Victoria. It is hardly one millionth part of the 

 distance of the stars, and thus the displacement of the 

 planet when viewed from the Cape and when viewed from 

 Europe is measurable. The measurements are made from 

 a comparison star with a micrometer or a helionieter. The 

 star is one beautifully-defined point, the planet is another, 

 and the difference of declination between the two can be 

 measured with a high degree of accuracy. Two comparison 

 stars symmetrically placed with regard to the planet are 

 usually employed, as a number of sources of error can be 

 thus eliminated. The displacement of the planet is to be 

 elicited by comparison between the observations in the 

 northern hemisphere with those in the southern hemi- 

 sphere. The observations must be made as nearly as 

 possible at the same time, due allowance being made for 

 the motion of the planet in the interval. 



Although every precaution is taken, yet the fact remains 

 that we are obliged to compare the measures made by 

 observers in the northern hemisphere with those made Viy 

 different observers, and, of course, with different instru- 

 ments in the southern hemisphere. In this respect we are, 

 no doubt, at no greater disadvantage than in the transit of 

 Venus; yet it is possible to obviate even this diliiculty, and 

 thus to give the minor planet method a great advantage 

 over its rival. The ditficulty would be overcome if we 

 could conceive that an observer and his observatory, after 

 making a set of observations on a fine night in one hemi- 

 sphere, were to be transferred, instrument and all, to the 

 other hemisphere, and then; to repeat the observations. 

 This problem can be solved without any miraculous agency, 

 and in it we have undoubtedly the most perfect mode of 

 measuring the sun's distance with which we are acquainted. 



This method has already been carried out with success 

 by Mr. Ciill in the case of Juno, but there are other planets 

 more favourably situated, and the work can be done for a 



