5 2 



NA TURE 



\_May 17, 1883 



nomical knowledge put in a very simple and taking form. 

 Thus, for instance, when Mr. Clark urges, and rightly 

 urges, that the latitude of any particular house where it 

 may be suggested to put up one of these instruments is 

 more likely to be accurately determined by a reference to 

 the Ordnance Map than in any other way, we are not 

 only told that the Ordnance Map may be got for is., but 

 the ignoramus is not even forgotten, and the way in which 

 longitude is marked on the map is clearly stated. Mr. 

 Clark it will be seen has spared no pains to make 

 everything as clear as possible to everybody. Here, for 

 instance, is what he says on counting time to a beginner 

 in astronomical work : — 



"In taking the time of transit by a watch some diffi- 

 culty will at first be experienced, owing to the fact that 

 watches tic'c various numbers of beats in a minute, but 

 rarely any direct multiple of the second. Having care- 

 fully ascertained the number, the watch is placed to the 

 ear at any even ten or twenty seconds, and the 

 counting is continued by sound, thus : One and, one 

 and, two and, two and, three and, three and, fifteen, 

 fifteen, &c. In recording the transit, the minutes and 

 seconds at the time of starting are noted, and the number 

 of additional ticks is counted by ear, and these are after- 

 wards converted into seconds and added. When a single 

 observation is considered sufficiently accurate for practical 

 purposes, it may be conveniently recorded by a chrono- 

 graph, which is started in exact accordance with your 

 clock a few minutes before it is required, and is stopped 

 at the exact instant of transit. At night this dispenses 

 with the necessity of referring to a lantern for time. 

 These chronographs may now be obtained at a very 

 moderate price, and when they beat quarter-seconds are 

 very useful for counting, and for carrying the time of a 

 clock or chronometer into the open air, and might be ad- 

 vantageously used on shipboard. The portable American 

 clocks, which are sold everywhere for a few shillings, 

 sometimes beat quarter-seconds, and serve well for count- 

 ing time. They may be easily converted into rough 

 chronographs by adding a stop movement in the form of 

 a light spring pressing against the balance wheel ; the 

 second hands are, however, loose and inaccurate. A 

 servant may be taught to record transits with the assist- 

 ance of one of these chronographs very correctly. 



" The writer, however, employs an arrangement which 

 he considers still preferable. A rough pendulum about 

 ten inches long is constructed out of a wire suspended by 

 a flat spring or by a double loop of wire, and screwed at 

 the bottom through a flat leaden plumb-bob ; it is 

 adjusted so as to beat half-seconds correctly. A very 

 small glass bead or button is suspended loosely at the 

 lower end of the rod, and strikes against a thin metal 

 plate at each oscillation ; it is started by a trigger of 

 wood or wire, supporting the plumb-bob at a definite 

 angle, and insuring uniformity of swing. This is fixed on 

 a board and hung within reach of the instrument. It 

 may be used in several ways ; the trigger may be pulled 

 at the instant of transit, the first tick being counted as 

 one, and the counting being continued while the ob- 

 server rises from his position, and obtains at leisure a 

 coincidence with a given second on his watch, the 

 number of beats being converted into seconds and 

 deducted from the time noted. The observation is noted 

 thus, 46m. 28-21, and is afterwards converted into 



8 46 



28 

 10-5 



8 46 17-5 



" Or the observer may start the pendulum just before 

 the transit occurs, and having obtained a coincidence with 

 his watch may continue to count by ear. The pendulum 



may, if preferred, be made to tick seconds by muffling 

 one side of the metal strip with a piece of felt, and it 

 may be removed after an observation, leaving the support 

 and striking plate attached to the pillars of the transit. 

 It oscillates on a rod or knife edge, and will continue in 

 motion for two or three minutes." 



As an appendix to the book we find transit tables 

 giving the Greenwich mean time transit of the sun and 

 certain stars for every day in the year, computed from 

 the Nautical Almanac, and there is also a table for con- 

 verting intervals of sidereal time into equivalent intervals 

 of mean solar time. More recently the author has pub- 

 lished a set of transit tables separately, and we would 

 recommend those who use his book on the transit instru- 

 ment to obtain them. To all who are not familiar with 

 the use of the tables, clear and simple instructions are 

 given, and we think they will prove of great use to those 

 for whom they are intended. 



Mr. Latimer Clark has certainly well deserved the 

 thanks of all interested in astronomy, for the pains he has 

 taken in thus endeavouring to popularise an instrument 

 which, although it is the most important instrument used 

 in astronomy for the determination of position, is at the 

 same time one from which an immense amount of 

 pleasure can be obtained by the merest tyro in science, 

 whilst the great advantage of using such an instrument 

 as this is, that no one can use it without rendering him- 

 self thoroughly familiar with some of the most important 

 problems which lie at the root of any useful knowledge 

 touching the stars, or the planet on which we dwell. 



LETTERS TO THE EDITOR 



[ The Editor docs not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return^ 

 or to correspond with the writers of, rejected manuscripts. 

 No notice is taken of anonymous communications. 



[The Editor urgently reqiusts correspondents to keep their letters 

 as short as possMc. The pressure on his space is so great 

 that it is impossible otherwise to insure the appearance even 

 of communications containing interesting and novel facts.~\ 



Fossil Algae 



In a review of Saporta's work on " Fossil Alijao " in Nature 

 (vol. xxvii. p. 514) there are certain opinions brought forward 

 which ought not to be passed by without some remarks. At 

 first it should be stated that Saporta, while still insisting upon 

 the vegetable nature of his so-called "algae," does not only 

 defend his views about those doubtful bodies which have been 

 the objects of my criticisms, but also "defends" true alga;, 

 concerning which I have never expressed any doubt. Such are, 

 for example, the Florideaa represented on the first three plates 

 in his work, and by "defending " them he puts me in a somewhat 

 false position, at least in the eyes of the readers who have not 

 studied my work. 1 The real fact is that I have only questioned 

 — and still do so — the vegetable nalure of almost all those objects 

 which Schimper in Zittel's " Handbuch der Paleontologie " com- 

 prises under the head of " Algie incerta sedis." There is con- 

 sequently some exaggeration in Saporta's statement of my 

 opinion. 



Now it is quite clear that fossil trails of animals must occur in 

 most cases in relief on the under tides of the slabs, the tracks 

 in such cases being impressions in the soft sand or mud, which 

 have since been filled by sediment. And it is also quite 

 natural that the trails of animals should especially be found 

 where there are alternating beds of sandstones and shales. 



Now it is a fact that the Bilobites, as well as Eophyton, always 

 occur in this way, projecting as convex bodies on the under sides 



1 A. G. Nathorst, " Om spar afnagra evertebrerade djur arbderas paleon- 

 tologiska betydelse." With a French resume, " Mcmoire sur quelques traces 

 d'animaux sans vertebres et de Ieur porttfe pale'ontuloglque." {Svttu&a 

 Vttemkaps Akademiens Hatidlingar, Bd.xviii.,No. 7. Stockholm: Norsted 

 och Soner, 1880.) 



