Dec. 1 8, 1 873 J 



NA TURE 



125 



the tables of these satellites should be brought to a high 

 degree of perfection ; and as, according to the opinion of 

 the most distinguished mathematicians, the observation 

 of all the phenomena which are presented by one of these 

 satellites in superior or inferior conjunction is the best 

 means of determining certain elements of the theory of 

 the satellites of Jupiter, it was useful to give in the col- 

 lection of ephemerides not only the epochs of the eclipses, 

 but also those of the contact of the shadow of the satellite 

 with the planet. Tables for the observation of the satel- 

 lites at the time of their maximum elongation would also 

 be very desirable. 



From the mariners' point of view, for whom the moon 

 is the principal heavenly body, the positions of the moon 

 calculated for noon and midnight of every day would be 

 insufficient on account of the considerable proper move- 

 ment of our satellite. To obtain the longitude of a place 

 by means of the observation of the passage across the 

 meridian of one of the limbs, there would be required an 

 excessively laborious calculation ; the use of that method, 

 however convenient, was then illusory. It was necessary 

 to give the right ascension and the declination for every 

 hour of the day, for the purpose of avoiding the employ- 

 ment of second differences except in cases where very great 

 precision was sought for. 



Then, when accurate tables of the movements of the 

 planets were obtained, it was useful to add to the distances 

 of the moon from the sun and from the stars, the distances 

 of that body from the principal planets, the observation 

 of which is more convenient and more certain than that 

 of its distances from the stars. 



But it was necessary to consider not only astronomers 

 in observatories and sailors on board their ships, it was 

 useful to enable astronomers on an expedition, and 

 sailors when in a foreign harbour, and also geographers, 

 to obtain the geographical co-ordinates of their station 

 with ease and accuracy. From this point of view the 

 method known as that of the Lunar Culminations holds 

 the first rank, a method to which a beautiful work by 

 Nicolai * gave a capital importance. The learned 

 director of the Observatory of Mannheim showed with 

 what facility the observations of the passage of the moon 

 combined with those of a certain number of stars, called 

 " stars of the moon," bordering on its parallel, and 

 passing the meridian a little before or a little after (half- 

 an-hour at the most), could give, sufficiently approximately, 

 the difference of the longitudes of two places, even with a 

 meridian instrument which was not perfect. On the other 

 hand, Bessel and Hansen had given simple methods for 

 calculating the horary movement of the moon. To apply 

 this method of lunar culminations, it was then necessary 

 to choose " stars of the moon," and to publish their posi- 

 tions every year, day by day, at the same time as those of 

 the moon at the moment of its passing the meridian. 

 This addition had, moreover, this advantage, that by indi- 

 cating by an asterisk the stars comprehended between 

 4° and 14" of declination, the observers of the two hemi- 

 spheres would have the elements most useful for im- 

 proving continuously (ii'iinc facon continnc) the value of 

 the lunar parallax. The phenomenon of the occultation 

 of the stars of the moon oft'ers, besides, an excellent 

 means of determining longitudes. It was then important 

 thus to calculate in advance and to publish all the 

 elements hkely to serve for predicting all the occultations 

 in a given place, for the purpose of rendering the employ- 

 ment of this method easy to the navigator. 



Finally it was indispensable, as well for the astrono- 

 mical operations of observatories as for those connected 

 with an astronomical or a geodetic expedition, that the 

 collection of ephemerides should contain, for epochs suffi- 

 ciently close to permit calculation for intermediate dates 



* "Uber die Melhode, Kingen durch Rectascensions-Differenzen ge-j 

 wahlten Vergleichsterne vom Monde zu bestiraraen" (^Astrouomisc/ie 

 ^atV(rjtA^t'« for 182^ and 1824.) 



by simple proportion, the apparent positions of a very 

 large number of stars of the greatest magnitude, and 

 distributed both in the north and south hemispheres. It 

 was useful, moreover, to join to this catalogue the values 

 for very close epochs of the constants of Bessel, which 

 enable one to pass from the mean position of a star at ths 

 commencement of the year to its apparent position on any 

 day whatever. 



For the principal circumpolars, a and S Ursas Minoris, 

 the importance of which is so great in determining the 

 various constants of a meridian instrument, and whose 

 apparent positions vary much more rapidly than those of 

 stars at a distance from the pole, — the apparent positions 

 ought to be given every day. 



Such is, with the exception of a few unimportant 

 details, the list of reforms which the general opinion of 

 astronomers demanded in England and Germany. 

 {To be continued^ 



ON THE SECONDARY WAVES IN THE 

 SPHYGMOGRAPH TRACE 



IN a letter printed in this journal a short time ago (vol. 

 viii. p. 464), Dr. Galabin refers to a paper which has 

 been since published in the journal of Anatomy and 

 Physiology (No. XII. p. i), for a fuller account of his 

 \iews as to the theory of the pulse, of which we gave a 

 short notice and criticism in a former number (vol. viii. 

 p. 330). This second and more detailed description calls 

 for further remark, especially as the author has found 

 reason somewhat to modify his opinion on one important 

 point. 



As is well known, the sphygmograph trace of a pulse 

 beat (see Fig. i) consists of a primary rapid rise, followed 

 by a more gradual fall, broken by a consideralale undu- 

 lation, termed the dicrotic wave, which varies in its dis- 

 tance from the next primary rise according to the rapid- 

 ity of the pulse. Between the primary and the dicrotic 

 rises in the trace, the descending curve is sometimes 

 interrupted by another small undulation termed the 

 "tidal" wave, by iVIr. Mahomed, tho;:gh the \-\an\e predi- 

 crotic is better, as it does not involve any theoretical 

 conceptions. It is the development in the trace of these 

 predicrotic and dicrotic waves that Dr. Galabin dis- 

 cusses and his explanation of the former is the fol- 

 lowing. — The separation of the primary and tidal 

 (predicrotic) waves is due to an oscillation in the 

 Sphygmograph, caused by the inertia of the instrument. 

 ... In some cases the lever may be separated slightly 

 from the knife-edge on which it rests, but generally the 

 oscillation takes place in the instrument as a whole, and 

 it may be followed by others in a descending series. 

 With reference to this interpretation, it may be first re- 

 marked that it seems almost impossible that the whole 

 sphygmograph should acquire a momentum in each pul- 

 sation, for it should be so adjusted on the arm that no 

 part except the tip of the spring is in any way in contact 

 with the artery, and when such is the case it is difficult to 

 conceive of any shock being communicated to the 

 whole. Again, any sudden upward impulse given to 

 the instrument itself would be attended with a descent 

 in the trace, for as the lever is only attached at one 

 end, and there only on points, its pen would be slow 

 to participate in the general movement of the framework, 

 and would not rise so rapidly as the recording paper. 

 The momentum acquired by the lever is a different thing. 

 Marey and Sanderson have both shown that the primary 

 rise in the trace may be attended with a sudden sharp- 

 pointed wave, in the production of which the lever leaves 

 the knife-edge on which it rests, returning to it after a 

 very short excursion. To prevent the excessive develop- 

 ment of this tmpC rfeel ion Marey has employed a small 

 secondary spring to depress the lever ; this spring Dr. 



