512 



NATURE 



[March 30, 1893 



meter directed towards the stern. Barometers can be placed in 

 this manner on ship board by the maker, and can be left to 

 themselves for any length of time. If the person to whom they 

 are consigned is notified of their subsequent arrival at port, he 

 can take them from their hangings on the ship in the best pos- 

 sible condition. Of course this presupposes an arrangement 

 with the officers of the vessel, such that the instruments shall be 

 let entirely alone from the time they are mounted by the con- 

 signor until they are received by the consignee. 



I think this method of carrying instruments can be very use- 

 fully applied in improving our knowledge of the relation of 

 international barometric standards, and at a minimum expense ; 

 and I will give a brief outline of a convenient way for accom- 

 plishing it. The Deutsche Seewarte at Hamburg, and the Kew 

 Observatory at Richmond (through the London Meteorological 

 Office), are in the best positions for supervising this work, and I 

 venture to express the hope that the matter will be seriously 

 considered. 



I will outline the work when carried on from London. 



Let two barometers of the best construction, say an Adie 

 Fortin and a VVild-Fuess control barometer, be compared with 

 the Kew normal during a period of a week or more, or long 

 enough to experience considerable variation in the barometer 



height. Then let the two barometers be mounted on one of the 

 London Hamburg steamships, in the manner which I have 

 described, and sent to Hamburg, where an employee of the 

 Deutsche Seewarte could be despatched to takedown the instru- 

 ments and carry them to the Seewarte for comparison with the 

 normal barometer. Then the barometers could be taken by a 

 messenger to Liibeck, at an expense of a few shillings, and 

 mounted on a St. Petersburg steamer, which would carry them 

 almost to the door of the Central Physical Observatory, where 

 they could be again taken in charge by a meteorologist, com- 

 pared for a few days, and then again be mounted on another 

 steamer bound for one of the Scandinavian ports where there is 

 a standard barometer, and finally returned to London by one 

 of the numerous regular steamships. At an expense of a couple 

 of pounds the barometer could be sent from St. Petersburg (or 

 Scandinavia) back to Hamburg via Stettin and Berlin ; thus 

 allowing Berlin to enter into the series. The barometers would 

 probably have to be sent by a messenger from Berlin to Ham- 

 burg, thus entailing the just mentioned expense. A second 

 comparison at Hamburg would be desirable, and then the 

 barometers could be returned to London by sea, and again com- 

 pared at Kew. 



Similarly, barometers could be sent to New York for comparison 

 with the sub-standard, by Adie, at the Maritime Exchange ; 

 although probably the United States Weather Bureau would 

 assume the expense of the three pounds necessary to carry the 



NO. 1222, VOL. 47] 



instruments to Washington for comparison with the normal 

 there, and then return them to New York and put them on ship- 

 board to be returned to London. 



The standard barometers of Australia, India, Brazil, and other 

 countries accessible by sea can be reached from London (or 

 Hamburg) in the same way, and the comparison instruments 

 can be returned to their starting point for additional verification. 



My own experience in the transportation of barometers assures 

 me that ship captains would gladly give their hearty co operation 

 to a work of this kind, and there would be no charges for carry- 

 ing the instruments even half round the world and back again. 



In offering this suggestion it is not necessary for me to give 

 the details for the complete organisation of such a scheme ; but 

 it may be remarked that if it should be undertaken, the personal 

 experience of those who have been over the ground should be 

 utilised in making plans. A single instance will serve to show 

 why this is advisable. Some years ago I carried two barometers 

 from Hamburg to London by sea. I took the German line of 

 steamers and found myself anchored in the middle of the Thames, 

 and bad to get ashore as best I could. I greatly feared that I 

 should never get the barometers ashore in a whole condition, 

 as there was necessitated a great deal of scrambling over lighters, 

 &c., and embarkation in an unsteady row boat in order to make 

 a landing. . Had I taken the English steamer, all this worry 

 would have been saved. Other similar instances occurred which 

 could have been avoided by one personally familiar with the 

 routes to be travelled. Frank Waldo. 



Princeton, New Jersey, February 20. 



Motion of a Solid Body in a Viscous Liquid. 

 There is perhaps no branch of mathematical physics which 

 has made greater progress during the last thirty-five years 

 than hydrodynamics. During this period numerous important 

 investigations have been published upon the motion of solid 

 bodies in 2ifriclionle.<;s liquid, upon the theory of discontinuous 

 motion, upon the theory of vortex motion and vortex rings, 

 upon the motion of a liquid ellipsoid under the influence of its 

 own attraction, and upon waves and tides. These investi- 

 gations constitute an enormous increase in the knowledge 

 possessed by the present generation compared with that of its 

 predecessors ; they have to a considerable extent exhausted the 

 field of research in the theory of the motion of frictionless 

 liquids ; but notwithstanding the importance of the results, the 

 elegance of the methods by which many of them have been 

 obtained, and the skill by which the mathematical difficulties 

 have been surmounted, all the investigations referred to possess 

 the defect of not accurately representing the motion of liquids 

 as they occur in nature. 



The reason of this discrepancy between theory and observation 

 is that the ideal substance, which is called a frictionless liquid, 

 has no actual existence, for all liquids which occur in nature are 

 viscous. The viscosity of the mobile liquids, such as water, 

 alcohol, &c., is a small quantity, being in thecase of water equal 

 to a tangential stress of about '014 dynes per square centimetre ; 

 whilst in the case of the sticky and greasy liquids, such as treacle 

 and oil, it is much greater. The viscosity of olive oil is about 

 3 '25 dynes per square centimetre, and is therefore about 232 

 times as great as that of water. 



The mathematical theory of the motion of viscous liquids was 

 elaborated as long ago as 184S by Sir G. Stokes, in a paper in 

 which he showed that the effect of viscosity might be represented 

 by certain additional terms in the equations of motion of a 

 frictionless liquid, which contain as a factor a new physical 

 quantity called the viscosity. In a subsequent paper, published 

 in 1850, he applied the above theory to calculate the diminution 

 of the amplitude of the small oscillations of a sphere surrounded 

 by v.'ater ; and by means of experiments in which this quantity 

 was observed, he calculated the numerical value of the viscosity 

 of water, and found that it was in close agreement with the 

 value found by Poiseulle from experiments on the flow of liquids 

 through capillary tubes. An investigation of a similar character 

 was undertaken by von Helmholtz and Piotrowski about 1863, 

 in which the sphere was suspended by a torsion fibre, and made 

 to perform small torsional oscillations about a diameter. 



Almost all calculations relating to small oscillations proceed 

 upon the basis that the squares and products of quantities, upon 

 which the disturbed motion depends, maybe neglected. This in- 

 troduces a great simplification into the work, and enables a variety 

 of problems, which would otherwise be exceedingly intractable, 



