June 21, 1888] 



NATURE 



185 



Messrs. D. C. Heath and Co. (Boston) will publish at 

 once Compayre's "Lectures on Pedagogy: Theoretical and 

 Practical," a companion volume to their Compayre's " History 

 of Pedagogy." It is translated and annotated by Prof. Payne, 

 of the University of Michigan. 



Prof. J. Violle has just issued the first part of the second 

 volume of his " Cours de Physique." The present part relates 

 to acoustics. 



We reprint from Science of June I, 1888, the following 

 suggestive paragraph : — "The Committee of the House of 

 Representatives on acoustics and ventilation has actually 

 reported favourably a Bill appropriating seventy-five thousand 

 dollars to subsidize a man who thinks he can construct a steel 

 • vacuum ' balloon of great power. He is to be allowed to use 

 the facilities of one of the navy-yards for the building of his 

 machine, and is to have the money as soon as he has expended 

 seventy-five thousand dollars of private capital upon his air-ship. 

 One of the mathematical physicists of Washington was asked 

 by a member of Congress whether such a balloon could be 

 successfully floated. He set to work upon the problem, and 

 here are some of his results, which are rather curious : — A 

 common balloon is filled with hydrogen gas, which, being lighter 

 than air, causes the balloon to rise and take up a load with it. 

 But, as the pressure of the gas within is equal to the pressure of 

 the atmosphere without, no provision other than a moderately 

 strong silk bag is required to prevent collapse. The inventor of 

 the proposed steel balloon hopes to gain greater lifting-power by 

 using a vacuum instead of gas, the absence of substance of any 

 kind being lighter than even hydrogen'gas. But he has to con- 

 tend with the tendency of the shell to collapse from the enormous 

 pressure of the atmosphere on the outside, which would not be 

 counterbalanced by anything inside of it. The first question 

 which presented itself was, How thick could the metal of the 

 shell be made, so that the buoyancy of the sphere, which would 

 be the most economical and the strongest form in which it could 

 be constructed, would just float it without lifting any load? 

 The computations showed that the thickness of the metal might 

 be "000055 of the radius of the shell. For example : if the 

 spherical shell was one hundred feet in diameter, the thickness 

 of the metal composing it could not be more than than one- 

 thirtieth of an inch, provided it had no braces. If it was 

 thicker, it would be too heavy to float. Now, if it had no 

 tendency to buckle, which of course it would, the strength of 

 the steel would have to be equivalent to a resistance of more 

 than 130,000 pounds to a square inch to resist absolute 

 crushing from the pressure of the air on a cross-section of the 

 metal. Steel of such high crushing-strength is not ductile, and 

 cannot be made into such a shell. If the balloon is to be braced 

 inside, as the inventor suggests, just as much metal as would be 

 used in constructing the braces would have to be subtracted from 

 the thickness of that composing the shell. Of course, such a 

 shell would buckle long before the thickness of the metal of 

 which it was composed was reduced to •000055 of its radius. In 

 other words, it is mathematically demonstrated that no steel 

 vacuum balloon could be constructed which could raise even its 

 own weight. This is an illustration of how intelligently Congress 

 would be likely to legislate on scientific matters unguided by 

 intelligent scientific advice." 



The additions to the Zoological Society's Gardens during 

 the past week include two Pig-tailed Monkeys (Macacus 

 nemestrinus i 9 ) from Java, presented by Mr. C. W. Ellacott ; 

 a Bonnet Monkey {Macacus sinicus 0. ) from India, presented by 

 Mr. J. Wiltshire ; a Pig-tailed Monkey (Macacus nemestrinus) 

 from Java, presented by Mrs. Gleig ; two Spotted Cavys 

 (Ccelogcnys paca 6 $ ) from South America, presented by Mr. 

 W. H. Stather ; a Mauge's Dasyure {Dasyurus maugai) from 



Australia, presented by Mr. H. R. Brame ; three Abyssinian 

 Sheep (Ovis aries, var.) from Abyssinia, presented by Mr. A. J. 

 Baker; two Pallas's Sand Grouse (Syrrhaptes paradoxus) from 

 the Island of Tiree, Argyllshire, presented by Lieut. -Colonel 

 Irby and Captain Savile Reid, F.Z.S. ; a Wapiti Deer (Cervus 

 canadensis 6 ), born in the Gardens. 



OUR ASTRONOMICAL COLUMN. 



The Constant of Aberration. — In the year 1862, Prof. 

 J. S. Hubbard commenced a series of observations of o Lyrae 

 with the prime vertical instrument of the Washington Naval 

 Observatory, which was continued by either Profs. Newcomb, 

 Harkness, or Hall until 1867. The purpose of these observa- 

 tions had been to obtain corrections to the assumed values of 

 the constants of nutation and aberration, and to afford an abso- 

 lute determination of the annual parallax of the star. The series 

 was not continued for a sufficient period for the first purpose ; 

 and Prof. Asaph Hall, when engaged on the determination of 

 the parallax of o Lyrae by another method, found that these 

 observations would give it a small negative value. From this 

 and other circumstances he was at that time induced to think 

 the observations would not repay the trouble of a careful dis- 

 cussion ; but recently, reflecting that they had been skilfully 

 designed, and carried out with care, he resolved to ascertain the 

 result they would furnish for the constant of aberration. The 

 observations commenced 1862 March 25, and extended to 1867 

 April 25, and were 436 in number. The mean resulting value 

 of the parallax is — 



tt — - o"-o79 ± o # "oi34, 

 whilst 



Constant of aberration = 20" -4506 ± o"x>i42, 



with an average probable error for a single observation of 

 ± o"-i74. 

 Adopting a parallax of + o'^lS, the result would be — 



Constant of aberration = 20" '4542 ± o"'OI44. 



Prof. Hall prefers this latter result, notwithstanding the un- 

 certainty as to the true parallax of the star. The negative 

 result obtained for the parallax may probably be due to the fact 

 that the coefficient of parallax obtains its extreme values in 

 January and July, when the mean temperature is likewise at its 

 extreme points ; the January observations also are made in day- 

 light, but the July at night, which would tend to produce a 

 systematic difference in the method of observing. The coefficient 

 of aberration, on the other hand, has its greatest values in April 

 and October, when the conditions of observation will be nearly 

 the same. 



The above value of the constant of aberration gives, for the 

 solar parallax — 



7r = 8" 810 ± o" - oo62, 



Hansen's values of the mean anomaly of the earth, and eccen- 

 tricity of its orbit being assumed, together with Clarke's value 

 for the equatorial radius, and Michelson and Newcomb's deter- 

 mination of the velocity of light, viz. 186,325 miles per second. 

 The Markings on Mars. — The observations of M. Perrotin 

 at Nice, and M. Terby at Louvain, and, in England, of Mr. Den- 

 ning at Bristol, have confirmed the presence on the planet of most 

 of the " canals " or narrow dark lines which were discovered 

 by M. Schiaparelli in 1877, and at subsequent oppositions. M. 

 Perrotin has also been able to detect, in several cases, the 

 gemination or doubling of the canals, and M. Terby has ob- 

 served the same phenomenon in one or two cases, but with 

 much greater difficulty than in the opposition of 1881-82. But 

 some curious changes of appearance have been noted. An 

 entire district (Schiaparelli's Lybia) has been merged in the 

 adjoining "sea,"z.*. its colour has changed from the reddish 

 hue of the Martial " continents " to the sombre tint of the " seas." 

 The district in question is larger than France. To the north of 

 this district a new canal has become visible, and again another 

 new canal has appeared to traverse the white North Polar cap, 

 or, according to M. Terby, to divide the true Polar cap from a 

 white spot of similar appearance a little to the south of it. With 

 the exception of these changes, the principal markings, both 

 light and dark, are those which former oppositions have rendered 

 familiar. 



