Sept. 28, 1876] 



NATURE 



479 



move without resistance in a perfect fluid ; and that thin surfaces 

 are no exception to the general rule, but I am not aware that 

 any satisfactory figure for the stream line of such bodies has as 

 yet been given. 



Now what I have to show raises two very important points in 

 connection with the stream-line theory, even as applied to a 

 perfect fluid. In the first place it will appear that a thin open 

 surface has no stream lines of its own (so to speak) except such 

 as it can claim as forming part of a closed surface. And in the 

 second place it will appear that the closed surface may assume 

 the form of a cylinder of indefinite length continually passing 

 away from the thin surface, in which case the surface or vane does 

 not move freely even through a frictionless fluid. If we place 

 a disc in front of one of these rings, the ring comes on until the 

 disc is against the bounding surface, and then carries the disc on 

 with it. It is certainly surprising to see a flat disc moving freely 

 through the water. I doubt not that the general impression is 

 that a thin flat disc is about the worst form of body to move 

 through water. And so it is, except when it has a vortex ring 

 behind it. 



Owing to the growth of the ring, if the speed of the disc be 

 maintained, the ring will gradually fall behind the disc, and the 

 disturbance caused in front will break it up. But if the disc be 

 allowed to move with the ring it will move freely as far as the 

 ring goes. A disc when first started forms its own ring. Thus if 

 a disc be floated on a light bar of wood, when the wood is drawn 

 forward at first, the disc offers considerable resistance to its 

 motion, but this resistance soon dies away, and if then the bar 

 be released, the disc will proceed steadily onward with a gradu- 

 ally diminishing velocity. 



A little colour in the water shows how the ring is formed and 

 how it moves onward behind the disc. 



The Resistance of an Inclined Vane. — The fact that the disc 

 will start its own ring, will close its own surface, is due to its 

 being symmetrical with respect to this surface. Half a disc will 

 not do it, much less any portion of the spheroid which was in- 

 clined to the front. 



When we draw a disc or flat surface edgewise through the 

 water it causes a continuous vortex cyl.nder which, forming at 

 the forward point of the vane passes away behind. The gyratory 

 motion of the water is somewhat disturbed by the friction of the 

 vane sliding past it, but by letting a little air down with the 

 vane the central lines of the vortices may be shown for several 

 feet in length. 



Having to form the vortices the forward edge encounters the 

 greatest resistance, and the whole resistance is steady and con- 

 tinuous. 



If my reasoning is right these facts are somewhat at variance 

 with the general notion as regards the results of the stream-line 

 theory, and at all events they furnish definite ideas of the results 

 we have to explain. It is, however, with the utmost diffidence 

 that I venture to bring forward my own explanation before such 

 an authoritative body as Section A. in the University of Glasgow, 

 and my chief object has been to illustrate the method of studying 

 fluid motion by observations on the motion of partly coloured 

 V ater. 



On the Protection of Buildings from Lightning, by Prof. J. 

 Clerk Maxwell. — Most of those who have given directions for 

 the construction of lightning-conductors have paid great attention 

 to the upper and lower extremities of the conductor. They 

 recommend that the upper extremity of the conductor should 

 extend somewhat above the highest part of the building to be 

 protected, and that it should terminate in a sharp point, and that 

 the lower extremity should be carried as far as possible into the 

 conducting strata of the ground, so as to "make" what tele- 

 graph engineers call "a good earth." 



The electrical effect of such an arrangement is to tap^ as it 

 were, the gathering charge by facilitating a quiet discharge be- 

 tween the atmospheric accumulation and the earth. The erec- 

 tion of the conductor will cause a somewhat greater number of 

 discharges to occur at the place than would have occurred if it 

 had not been erected ; but each of these discharges will be 

 smaller than those which would have occurred without the con- 

 ductor. It is probable, also, that fewer discharges \\nll occur in 

 tihe region surrounding the conductor. 



It appears to me that these arrangements are calculated rather 

 for the benefit of the surrounding country and for the relief of 

 clouds labouring under an accumulation of electricity, than for 

 the protection of the building on which the conductor is erected. 



What we really wish is to prevent the possibility of an electric 

 discharge taking place within a certain region, say in the inside 



of a gunpowder manufactory. If this is clearly laid down as our 

 object, the method of securing it is equally clear. 



An electric discharge cannot occur between two bodies, unless 

 the difference of their potentials is sufficiently great, compared 

 with the distance between them. If, therefore, we can keep the 

 potentials of all bodies within a certain region equal, or nearly 

 equal, no discharge will take place between them. We may 

 secure this by connecting all these bodies by means of good con- 

 ductors, such as copper wire ropes, but it is not necessary to do 

 so, for it may be shown by experiment that if every part of the 

 surface surrounding a certain region is at the same potential, 

 every point within that region must be at the same potential, 

 provided no charged body is placed within the region. 



It would, therefore, be sufficient to surround our powder-mill 

 with a conducting material, to sheathe its roof, walls, and ground- 

 floor with thick sheet- copper, and then no electrical effect could 

 occur within it on account of any thunderstorm outside. There 

 would be no need of any earth connection. We might even 

 place a layer of asphalte between the copper floor and the 

 ground, so as to insulate the building. If the mill were then 

 struck with lightning, it would remain charged for some time, and 

 a person standing on the ground outside and touching the wall 

 might receive a shock, but no electrical effect would be perceived 

 inside, even on the most delicate electrometer. The potential of 

 everything inside with respect to the earth would be suddenly 

 raised or lowered as the case might be, but electric potential is 

 not a physical condition, but only a mathematical conception, 

 so that no physical effect would be perceived. 



It is therefore not necessary to connect large masses of metal 

 such as engines, tanks, &c., to the walls, if they are entirely 

 within the building. If, however, any conductor, such as a tele- 

 graph wire or a metallic supply-pipe for water or gas comes into 

 the building from without, the potential of this conductor may be 

 different from that of the building, unless it is connected with 

 the conducting-shell of the building. Hence the water or gas 

 supply pipes, if any enter the building, must he connected to the 

 system of lightning conductors, and since to connect a telegraph 

 wire with the conductor would render the telegraph useless, no 

 telegraph from without should be allowed to enter a powder- 

 mill, though there maybe electric bells and other telegraphic ap- 

 paratus entirely within the building. 



I have supposed the powder-mill to be entirely sheathed in 

 thick sheet co pper. This, however, is by no means necessary in 

 order to prevent any sensible electrical effect taking place within 

 it, supposing it struck by lightning. It is quite sufficient to indole 

 the building with a network of a good conducting substance. 

 For instance, if a copper wire, say No. 4, B.W.G. (0-238 

 inches diameter), were carried round the foundation of the house, 

 up each of the comers and gables and along the ridges, this 

 would probably be a sufficient piotection for an ordinary build- 

 ing against any thunderstorm in this climate. The copper wire 

 may be built into the wall to prevent theft, but should be con- 

 nected to any outside metal such as lead or zinc on the roof, and 

 to metal rain-water pipes. In the case of a powder-mill it might 

 be advisable to make the network closer by carrying one or two 

 additional wires over the roof and down the walls to the wire at 

 the foundation. If there are water or gas-pipes which enter the 

 building from without, these must be connected with, the system 

 of conducting-wires, but if there are no such metallic connections 

 with distant points, it is not necessary to take any pains to facilitate 

 the escape of the electricity into the earth. 



Still less is it advisable to erect a tall conductor with a sharp 

 point in order to relieve the thunder-clouds of their charge. 



It is hardly necessary to add, that it is not advisable, during a 

 thunderstorm, to stand on the roof of a house so protected, or to 

 stand on the ground outside and lean against the wall. 



On a Cyclone Periodicity, in connection ivith Sun-sfot Periodi- 

 city, by C. Meldrum. — This paper is a continuation of the one 

 published in the report for 1874 ; it contains a discussion of the 

 cyclones that occurred in the Indian Ocean, from the equator to 

 32° S. and 0° to 120° E., in the years 1868-75. From 1868 to 

 1872 the cyclonic area increased, and since 1872 it has been 

 decreasing. In 1868 it was two millions of square miles, in 

 1872 between four and five millions, and in 1875 nearly two 

 millions. The rainfall over the globe generally seems to have 

 had a similar march, the rainiest year being 1872. 



Mr. O. J. Lodge exhibited diagrams of a model to illustrate 

 mechanically the passage of electricity through metals, electro- 

 lytes, and dielectrics, according to Maxwell's theory. The model 

 consisted of an endless cord passing with friction through 

 buttons supported on elastic strings, and by altering the relation 



