604 



NA TURE 



{Oct. 1 8. 1883 



system as this is adoptei in reading all large circle*. But 

 when it is a question of measuring smaller arcs, the micrometer 

 may be used with the telescope itself, its wires appearing wilh 

 the image of the object in the field of view. 



A description of an experiment will perhaps convey a better 

 idea of what can be done in this way. Fig. 15 represents the 

 arrangement. The condensing lens of the lantern having been 

 removed, the light is allowed to impinge upon a lens, A, placed 

 at a slight distance from the lantern. Its action on the light 

 causey a reversed image of the poles to be produced in the air. 

 The light coming from this image is then made to pass through 

 another lens, b ; the reversal is corrected, and a magnified image 

 of the poles of the electric arc is thrown upon the screen. The 

 first lens which forms the image may be regarded ns the object- 

 gliss of a telescope, whilst the other lens which throws the mag- 

 nified image upon the screen is the counterpart of the telescope's 

 eyepiece. Now if at c, where the image is formed in air, the 

 micrometer wires are placed, they, with the image of the poles, 

 will appear magnified on the screen. 



In this manner bodies appear with the wires in the field of 

 vision of the telescope, and their diameters and the dimensions 

 of different parts of them may be most accurately determined. 

 Up to the present time we have been concerned simply with 

 accurately determining the positions occupied by the various 

 bodies which people space. But with this micrometer in the 

 field of view of the telescope something more than this may he 

 done. We may now determine some measurements upon the 

 bodies whose positions alone we have been considering up to 

 now. For instance, the image of a planet may be grasped by 

 the wires, one wire bounding one limb of the planet, the other 

 wire lying along the other limb. Then, knowing how many 

 complete turns, and i/tooth, and l/ioooth parts of a turn have 

 been given to the head of a screw in order that the wires may be 

 separated through such a distance, and knowing also the value 

 of these divisions in seconds of arc, the diameter of the planet 

 may be measured. In like manner, the heights of lunar moun- 

 tains may be ascertained by measuring the lengths of the 

 shadows thrown by them. Or it may be a que-tion of the 

 distance between two stars close together. The method is still 

 the same. One star is made to lie along the movable wire, the 

 other is seen on the fixed one, and the distance through which 

 the wires are separated ascertained. Having attained to this, 

 let us bring our inquiry into angular measurement to a close. 



In passing, as we shall in the sequel, from the measure- 

 ment of space to the measurement of time, it will be found that 

 the difficulties have been grappled with very much in the same 

 way. In this measurement of space we began with simple instru- 

 ments, and only by a slow growth has the modern instrument 

 been arrived at ; yet notwithstanding the immense changes that 

 have taken place, the pointer and circle of the old instrument are- 

 still represented in the new, whilst the vernier and micrometer, 

 still preeminent, enable a degree of fineness to be attained quite 

 unparalleled in the old days. But in passing from these older 

 instruments, in which the circle was so prominent a feature, and 

 the pointer so small, to the more modern instruments, it will be 

 seen that, although both are still preserved, a great change has 

 taken place. The pointer, represented by the telescope, is the 

 prominent part of the instrument, whilst the circle is hidden away 

 almost out of sight. J. Norman Lockyer 



(To be continued.) 



TABLE OF DIFFERENT VELOCITIES 

 EXPRESSED IN METRES PER SECOND 



T*HE following table has been drawn up by Mr. James 

 Jackson, Librarian to the Paris Geographical Society : — 



Metres per 

 second. 

 A man walking 4 kilometres an hour I'll 



.. ». 5 ,. „ I'4° 



The comet of Halley in aphelion 3-25 



A ship going 9 kn >ts an hour (9 x 1852 metres) 4-63 



Ordinary wind from 5 to 6 



A ship going 12 knots an hour (12 x 1852 metres) ... 617 

 A wave 30 metres in magnitude with a depth of 300 



metres 6"S( 



A ship going 17 knots an hour (17 x 1852 metres) ... 875 



A fresh breeze 10 



A torpedo boat going 2 1 knots an hour (21 x 1852 metres) 10S0 



Metres per 



second 



12 



'5 



16-67 

 18 

 21 85 



A race-horse trotting an English mile in 2 mill. 14 sec. 



,, ,, galloping 900 metres a minute 



An express train running 60 kilometres an hour 



Flight of a falcon, of a carrier pigeon 



A wave in a tempest at sea 



An express train running 60 English miles an hour 



(60 x 1609 metres) 



A tempest from 25 to 30 



The transmission of sensation by human nerves 33 



A hurricane 40 



Flight of one of the swiftest birds 88' 



Velocity of a point on the equator of Mercury 



Propagation of the tide caused by the earthquake of 



Arica on August 13, 1868 (Arica to Honolulu), 



according to Hochstetter 



Velocity of a point on the equator of Mars ... 



,, sound in the air ( + 10° C.) 



,, a point on the equator of Venus... 

 „ „ ,, the Earth 



A cannon ball 



Propagation of the movement of tides (North Pacific 



Ocean) : maximum according to Whewell 



The moon's revolution round the Earth 



Velocity of a point on the equator of Mercury 

 Revolution of the second satellite of Mars ... 

 Concussion of the earthquake of Viege duly 25, 1855) 



from Turin to Geneva in 126 seconds 



Velocity of sound in water ( + 8°'I C.) 



Revolution of the first satellite of Mars 



Velocity of a point on the equator of the Sun 

 Revolution of the fourth satellite of Uranus .. 



,, ., eighth ,, Saturn .. 



,, ,, thirl ,, Uranus.. 

 Velocity of a point on the equator of Uranus 

 Revolution of the satellite of Neptune 



,, ,. second satellite of Uranus.. 

 of Neptune round the Sun 



., of the first satellite of Urnnus 



,, .. seventh „ Saturn 



,, .. sixth .. „ 



,, of Urmus round he Sun 



Proper movement of Ye_:a 



Displacement of the sun towards the constellation of 



Hercules 7642 



Revolution of the fourth satellite of Jupiter 8359 



„ of Saturn round the Sun 9584 



,, of the fifth satellite of Saturn 974' 



Velocity of a point on the equator of Saturn 10,541 



Revolution of the third satellite of Jupiter 10,869 



,, fourth „ Saturn ",5'6 



Velocity of a point on the equati r of Jupiter ... 12,491 

 Revolution of Jupiter round the Sun 12,924 



,, of the third satellite of Saturn 13,038 



,, ,, sec mhI ,, Jupiter '3,999 



,, „ ,. „ Saturn 14.568 



,, first ,, „ 16,425 



„ „ Jupiter 17.667 



., of Mars round the Sun 23.863 



., of the Earth ,, ,, 29,516 



,, of Venus ,, ,, 34-d30 



Troper movement of Capel la 4o,roo 



Revolution of Mercury round the Sun 47. ? 27 



Proper movement of Sirius 51,000 



Ordinary movements of the solar 



2681 



8S"9o 

 14687 



22738 



244 

 337 20 



454-58 



463 



500 



922 

 1012 

 i°34 

 "57 



J368 

 '435 

 '833 

 2028 

 3300 

 3738 

 3814 

 3904 

 4505 

 4906 



5390 

 5763 

 5794 

 6398 

 6730 

 7000 



Proper movement of the 6 1st of Cygnus 



,, .. Arcturus 



The comet of Halley in perihelion 



Tempests of the solar atmosphere 



Electricity ; a telegraphic submarine wire 



„ ,, atrial 

 Velocity of light 



atmosphere 



from 30,000 to 65,000 



71,600 



85,000 



... 393,260 



... 402,000 



... 4,000,000 



36,000,000 



300,400,000 



THE BRITISH ASSOCIATION 



REPORTS 



Report 0/ the Committee on Electric Standards, read by Mr. 

 R. T. Glazebrook. — This comprised an account of the means 



