Sept. 27, 1877] 



NATURE 



457 



/ quarters, an impression not fair to him, and as the same accounts 

 may produce the same impression abroad, it seems proper to 

 make the following statement : — 



When on the morning of August 17 Prof. Hall showed 

 me his obser\'ations, ttie communication was purely con- 

 fidential and friendly, and was not made either in the hne 

 of duty or because he failed to recognise the significance 

 of his observations, or because any special skill he did not 

 possess would aid in interpreting them. I suggested that, 

 from the few measures he had made, it was possible to 

 estimate the time of revolution of the satellite, if the object really 

 were one ; and thus ventured the prediction that it would be 

 hidden during most of the following night, but would reappear 

 toward morning near the position in which it was seen the night 

 before. The fulfilment of this prediction facilitated the esta- 

 blishment of the true character of the object, but, without it, an 

 equally certain hold on the satellite would very soon have been 

 obtained by Prof. Hall alone. The credit of sole discoverer is 

 , therefore due to him. Simon Nevvcomb 



The Satellites of Mars 



It may interest some of the readers of Nature to know that 

 one of the recently-discovered satellites of Mars appears to have 

 been certainly seen with the six-f lot reflector at Parsonstown. 

 My assistant writes me on the 17th instant : — 



" On the Sth instant (before receiving the Washington circular) 

 I suspected very strongly at 1 1 "45 P. M, while using the six- 

 foot, that a satellite was visible, following the disc, about ij 

 diameters. It appears now from the eleaients that it must have 

 been the outer one. On the 15th instant, at 11 30 p.m., I saw 

 it quite distinclly preceding the planet, however not well 

 enough to measure it, as I lost it again after a couple of minutes, 

 owing to the strong glare of Mars. List night I saw it again, 

 but only by glimpses, twice or three times." 



The unfavourable weather prevented the satellites from being 

 looked for between the Sth and 15th instant. 



I may add that it seems probable that the satellites might 

 have been measurable on the 15th instint with the bright hne 

 micrometer had it not been in the maker's hands. The low 

 meridian altitude of the planet (25'' at Parsonstown as compared 

 with 40° at Washington) is of course a serious drawback to 

 observations at the former place. RossE 



Yorkshire, September 20 



The weather in this neighbourhood has been very unfavour- 

 able for observation ever since the announcement of the discovery 

 of two satellites of Mars. Last night, however (about 9.30), 

 during little more than half-an-hour's interval of clear sky, the 

 air being extrtmely steady, and the planet beautifully defined, I 

 succeeded in seeing the outer satellite of the two. With the full 

 aperture of my 18-inch silvered-glass equatorial reflector, and 

 an ordinary achromatic eye-piece with a bar across the field 

 hiding the planet, the satellite was but glimpsed occasionally ; 

 with a single double-concave lens (power about iSo) it was 

 visible, in spite of the brilliant light of the planet. Had I not 

 known its exact position, however, I question whether I should 

 have seen it at all. It is a most difficult object. 



Henry Cooper Key 



Stretton Rectory, Hereford, September 19 



A Good Suggestion 



The approaching meeting in London of librarians represent- 

 ing the most important English, and, I believe, foreign, collections 

 of books, makes the present a suitable time to ofler suggestions 

 as to the management of such collections. 



It has long seemed to me that an improvement might be 

 made of a very simple nature, but capable of greatly increasing 

 the working value of reference libraries, especially those of the 

 first rank ; namely, to provide, somewhat as follows, for their 

 being consulted by those who cannot personally visit them. 



Suppose that the authorities of such an institution as the 

 British Museum or the Bodleian designate certain persons, not 

 paid officers of the library, but known to its directors as well- 

 educated, trustworthy, and acquainted with the resources of the 

 particular library ; publishing the names and addresses of these 

 gentlemen as willing, and believed to be competent, to under- 

 take researches amongst the bo..ki for those who may write to 

 them from a distance ; the official authorities assuming no actual 



responsibility for the work so done, but merely recommending 

 the persons to do it ; publishing at the same time a definite 

 statement of the payment expected per day or hour by these 

 persons. 



Often when one would desire to consult a great public library 

 in a foreign land, or in a distant part of one's own country, 

 nothing short of a personal visit would be of use, but in very 

 many cases it would be quite possible to obtain all that one 

 desired by a simple business-like correspondence with a proper 

 agent. Sometimes the quesiion is merely whether such or such 

 a book exists in the library, with perhaps an accura'e copy of its 

 title ; sometimes a special reference to a single page in an old 

 and scarce scientific journal or set of transactions is to be veri- 

 fied ; sometimes a few paragraphs are to be copied in tlie exact 

 words of the author ; sometimes a name, date, or number is to 

 be sought out ; sometimes a larger amount of work would be 

 needed, but so definitely shaped out that instructions in writing 

 could easily be given for it to an inteUigent person on the spot. 

 As it is, the consulting of such a distant library in person is often 

 simply impossible, and even when possible, often involves such 

 expense and delay as to make themselves seriously felt ; whereas 

 by the plan proposed, the object in view might often be attained at 

 a cost of time and money altogether trifling. In my own very small 

 experience I once found it necessary to travel some 700 miles, 

 losing three days, and spending about 7/., in order to refer to a 

 book for about ten minutes, while directions for m iking the same 

 search could have easily been put upon half a sheet of note- 

 paper, and carrying them out would have occupied a person 

 living in the city in which the library was situated altogether not 

 more than an hour or an hour and-a-hilf. 



In the neighbourhood of almost every large library competent 

 men might reidily be found to undertake such work as is 

 suggested, and to whom the opportunity of increasing their 

 income, or probably in time earning from this source alone a 

 satisfactory income, would be welcome. The plan would admit 

 of being carried out upon a small or an extended scale ; a library 

 of the ttiird or fourth class might afford a field for a single man 

 only, while one of the first class would be likely gradually to 

 enlist the services of a number ; if this were so sub-division of 

 labour would be desirable, one person undertaking researches in 

 natural history, another in mathematics, physics, or chemistry, 

 another in classical learning, &c. 



While such work could not properly be done by the regular offi- 

 cers of a public library, it would be important tliat the private indi- 

 viduals who were to enter upon it should have the approval of, 

 and should be recommended by, the library authorities, who 

 might also very properly fix the rate of payment, recommending 

 only those who were willing to accept the rules laid down. 



This plan has at least the merit that it might be tested with 

 very little trouble, risk, or disturbance of existing arrangements. 

 I believe that even in England with great libraries situated at 

 comparatively moderate distances from almost every one in the 

 kingdom, it would prove a great convenience ; to persons placed 

 as are those who live here in America, with nolibrary of the first 

 rank on this side of the ocean, and with hundreds of miles often 

 separating one from the largerof even those libraries which do here 

 exist, the boon of access by letter to the greatest collections ol 

 the world would be inestimable. It would be in a new direction, 

 and a noble one, carrying out the tendency of the most modern 

 civilisation which looks to placing, as far as possible, the 

 resources of the whole earth within the reach of hini who lives 

 at any one spot upon its surface J, W. M/VLLET 



University of Virginia, Setember 5 



Some of the Troubles of John O'Toole respecting 

 Potential Energy' 



IL 



B. — Potential E. , as meaning ' ' energy related to Potential 

 Fimetions." 



We now pass to the second meaning of "potential E." 

 It happens, by a most singular and unfortunate coincidence, that 

 this class of £. can very well be called by that title for a reason 

 quite distinct from that which we have been deprecating. The 

 idea of the potential function, or briefly, potential, was first 

 formed and thus named l)y Green. It has no reference whatever 

 to exi-ting in possibility ; it is concerned with present potency 

 or power ; and it happens that potential E. of unit of mass may 



' Continued from p. 441. 



