538 



NA TUKE 



[April 4 1901 



In an introduction a brief historical survey is given of 

 our knov^fledge of the Jurassic plants of Yorkshire, and 

 also of the Jurassic plant-bearing strata of France, 

 Germany and other countries which resemble those of 

 the Yorkshire coast. 



In the descriptive part of the catalogue the specimens 

 are grouped, so far as possible, in accordance with 

 their natural affinities. Fifty-five species are described, 

 and are distributed as follows : — Bryophyta, i ; Equi- 

 setales, 2 ; Filices, 20 ; Cycadales, 23 ; Coniferit, 9. 

 There is a resemblance between this flora and the 

 Wealden flora, among the common characteristics being 

 the absence of Angiosperms and abundance of Cycads 

 and Ferns. 



In conclusion, Mr. Seward remarks : " It is in the 

 southern tropics that we must look for existing forms 

 which afford the most striking links between the vege- 

 tation of to-day and that which has left imperfect records 

 in the Jurassic sediments of the Yorkshire coast. The 

 climate was presumably more tropical than that of North 

 Europe at the present day ; there is no evidence that the 

 plants of Jurassic times grew under conditions which 

 induced xerophytic characters, moisture being probably 

 abundant and favourable to the luxuriant growth of 

 Equisetums and Ferns." 



Practical Electrical Testing in Physics and Electrical 

 Engineering. By G. D. A. Parr. Pp. vi -I- 392. 

 (London : Longmans and Co., 1901.) Price 8^-. bd. 



Mr. Parr is head of the electrical engineering depart- 

 ment of Yorkshire College, Leeds, and the book before us 

 represents the instructions and 

 experiments given to students in 

 the practical course in that de- 

 partment. It is not our object to 

 criticise that course, the value of 

 which must largely depend on the 

 theoretical teaching accompanying 

 it ; suffice it to say that it is appa- 

 rently modelled very closely on 

 that given by Prof. Ayrton at the 

 CentralTechnical College. Whether 

 there is sufficient justification for 

 the publication of Mr. Parr's book 

 must depend on whether the details 

 of the system and apparatus are 

 sufficiently widespread ; in the 

 majority of cases the instructions 

 given for each experiment imply 

 the provision of special apparatus 

 for carrying out the test, and the BouRNtMouT« 

 instructions are given, if we may 

 use the phrase, in terms of 

 that apparatus. A more general 



description of the experiments would be of wider use, 

 though probably not so convenient for the student actually 

 passing through Mr. Parr's course. There is a good 

 appendix describing the chief instruments and apparatus 

 used, and another appendix giving solutions of the various 

 problems raised by the experiments which we do not 

 think so valuable, except as a labour-saving device to the 

 demonstrator or idle student. The experiments are, on 

 the whole, well devised to bring out clearly the funda- 

 mental laws of electricity and magnetism. We are sorry, 

 however, to see included an experiment to " prove " Ohm's 

 law, in which P.D. is measured by a high-resistance 

 galvanometer ; a galvanometer can only be employed to 

 measure P.D. if Ohm's law be true, so that it cannot 

 logically be used for this purpose in an experiment to 

 prove the law. The method given second by Mr. Parr, 

 m which an electrometer is used to measure P.D., is the 

 only satisfactory one for proving Ohm's law. Technical 

 teachers who are seeking to develop a practical course 

 will find this volume a valuable guide. 



NO. 1640, VOL. 63] 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications,^ 



Audibility of the Sound of Firing on February i. 



It is an interesting question how far the accounts of 

 various observers as to the sound of the minute guns on 

 February i having been distinctly heard at many distant 

 stations and not heard at others comparatively near by attentive 

 listeners, and as to the character and duration of the sound, can 

 be explained by known laws of the propagation of sound in the 

 atmosphere. To this question I should like here to offer such 

 answer as I have been able to arrive at after careful considera- 

 tion and some rough calculation. 



The firing line extended from the Majestic at the eastern en d 

 to the Alexandra at the western, in a direction some 6° N. of 

 W. for about 8 miles. The eastern half was a double line of 

 16 pairs of ships, the distance between the two lines being about 

 \ mile, nearly the same as that between the successive ships in 

 each line (2^ cables or \ sea mile) ; while the western half was a 

 single line of 14 ships. 



Some stress has been laid by observers near the firing line on 

 the want of simultaneity in the discharges from the different ships. 

 There is doubtless need of more accurate information on this 

 point, but I cannot help thinking Mr. Hinks's estimate that, 

 as "the firing ran down the double line, the interval between 

 the successive pairs of flashes was about half a second," is ex- 

 cessive. This would add, to an observer at Southsea, 8 seconds 

 over and above the 19 to 20 seconds by which the sound of the 

 westernmost pair of the double lin e was necessarily behind 



WINCHESTER. 



cmCHceTCR 



Fig. 



that of the first pair, whereas he says that the whole sound 

 "lasted only about 20 seconds." I do not think, therefore, in 

 the absence of more exact information, account need be taken in 

 a general explanation of the supposed want of simultaneity, 

 which I estimated on such information as I could get as not 

 amounting to a difference of more than 2 or 3 seconds. 



Mr. Hinks's observation is in another respect important, 

 as it would seem to show that the second half of the firing line 

 was unheard at the distance of 4 to 5 miles from its eastern end. 



The known causes which would or might affect the audibility 

 of the sound impulses arriving at any station are, I think, 

 these : the position of the station relatively to the firing line, the 

 direction of the wind and the variation in its speed with the 

 height above the earth's surface, the variation in temperature of 

 the different strata of the atmosphere, and for greater distances 

 the curvature of the earth's surface. 



Of these, for distances not exceeding 30 to 35 miles from the 

 firing line, I believe the dominant factor to be the first, that is, 

 relative position of the station. 



The accompanying diagram (Fig. i), drawn roughly to scale, 

 shows this, I think, conclusively. 



At Shanklin the reports were heard, but they were not very 



