lOO 



NA TURE 



[July 21, 1923 



that would be employed if the sea were about 4000 

 miles deep. But the sea is only about 2 miles deep, 

 in which, obviously, the tide would be practically 

 insensible compared with that in a sea 4000 miles 

 deep. The height of the tide is measured from the 

 sea bottom ; so that whatever elevation by tidal 

 action of that bottom, which takes place, is not added 

 to the height of the tide at all. Why, then, should 

 the action of the tide-raising force beneath the 

 bottom of the sea be added to that action within 

 the sea itself (as is invariably done) to obtain, or 

 explain, the ocean tide ? 



There are many other points of the tidal theory 

 discussed in the pamphlet referred to which are 

 equally difficult of explanation according to the 

 theory of gravitation. To a statement of these 

 difficulties it is really a rather unsatisfactory answer 

 to say merely that their presentation betrays " a 

 complete misunderstanding of the theory." Facts 

 should be given to show of just what the misunder- 

 standing consists ; and truly such facts would not 

 be a waste even of the columns of Nature, but would 

 undoubtedly prove edifying to many of your readers 

 besides the present writer. 



The " Tides " pamphlet will be sent free on request 

 to any one interested. Evan M'Lennan. 



Corvallis, Oregon, U.S.A., 

 May 26. 



To any one who understands the theory of gravita- 

 tion, Mr. M'Lennan's letter is a complete justification 

 of the note. To those who would think that there 

 might possibly be something in it, nothing less than 

 chapter v. of Sir G. H. Darwin's popular book on 

 ^ tides would be of any use. There are too many 

 important and pressing demands upon the space of 

 Nature to permit a full discussion of the points put 

 forward by Mr. M'Lennan. We can only remark 

 that the tides are due to the difference between the 

 response of the oceans and the solid earth to the 

 attractions of the sun and moon. The motion of the 

 solid earth as a whole is determined by the forces at 

 its centre, so that the differential motion of the 

 oceans is determined by the vectorial excess of the 

 forces at the earth's surface over those at its centre. 

 Of this excess it is the component tangential to the 

 earth's surface which is effective in producing the 

 tides. The Writer of the Note. 



Barometric Pressure in High Latitudes. 



In his letter (Nature, May 12, p. 634) on the 

 subject of the causation of anticyclones, recently 

 under discussion, Mr. R. M. Deeley makes two 

 statements which cannot, on the most liberal inter- 

 pretation of their face value, be reconciled with the 

 real facts of the case as they are well known to 

 meteorologists. 



First of all he says : " Another clear effect of 

 surface temperature is the fact that the North Pacific 

 cyclone and the North Atlantic cyclone (the eyes of 

 the North Polar cyclone) are more powerful during 

 the summer than they are during the winter." 

 This is in direct opposition to the truth, as any one 

 will find who refers to charts of mean pressure for 

 January and July, wherein he will find the Icelandic 

 and Bering Sea minima greatly accentuated in winter 

 and nearly obliterated in summer. Moreover, these 

 mean or average charts are merely the generalised 

 expression of one of the most obtrusive facts of 

 seasonal climatology ; namely, the frequently violent 

 cyclonic mood of the North Atlantic ocean in mid- 

 winter, and its generally much milder state at mid- 

 summer, together with the many more I gales we 



NO. 2803, VOL. 112] 



experience in England in December and January than 

 in June and July. 



Secondly, Mr. Deeley refers to " the striking facts 

 that throughout the year the great low-pressure areas 

 are over the frigid poles." Now though there may 

 be relatively low pressure with cyclonic circulation 

 at higher atmospheric levels round the p)oles, the 

 modern work of Dr. G. C. Simpson for the Antarctic, 

 and of Prof. Mohn for the Arctic, indicate that the 

 surface pressure at both p>oles is relatively high, 

 supplying an outflow of air towards the low pressure 

 belts about latitudes 60° N. and S. In the Antarctic 

 there is a true glacial anticyclone ; in the Arctic the 

 land areas round the polar basin complicate the 

 distribution of pressure, but the pressure over that 

 basin is relatively high throughout the year, parti- 

 cularly in winter, when it links the interior glacial 

 anticyclone of Greenland with the continental anti- 

 cyclone of Siberia. 



Moreover, if the Polar Front theory of Prof. 

 Bjerknes is true — and though there are justifiable 

 doubts as to whether that theory is a full dynamical 

 explanation of cyclonic circulation, no weather fore- 

 caster will dispute that it provides an excellent geo- 

 graphical background of reference for the facts associ- 

 ated with that circulation — there must, on the average, 

 be relatively high surface pressure about the poles. 



With regard to the effect of surface temperature 

 on pressure it is quite true (as Mr, Deeley observes) 

 that in the northern hemisphere, where there are 

 such violent contrasts of continent and ocean, the 

 continents command the excess of air in winter on 

 account of the cold, but lose it to the oceans in 

 summer on account of the heat. But this relation- 

 ship between surface temperature and pressure is 

 only very rough. There cannot be high or low 

 pressure everywhere ; and the actual result is a 

 highly complicated regional compromise. If the 

 northern hemisphere were all land or all water, 

 there could not be those marked seasonal or monsoonal 

 disturbances, so conspicuous on the January and 

 July charts of mean pressure, of the simple dynamic 

 belts of wind and pressure, namely, low at the equator, 

 high at about 30° N. and S., low again at about 

 60° N. and S., high again at the poles, to which one 

 gets an approximation on the annual chart and 

 also on those for April and October. One must 

 grant that the circulation of the atmosphere is 

 initiated and maintained by the general thermal 

 gradient between the equator and the poles ; but 

 the rotation of the earth and the seasonal contrasts 

 of temperature between continents and oceans com- 

 bine to impose an exceedingly complex structure upon 

 the circulation. L. C. W. Bonacina. 



27 Tanza Road, Hampstead, N.W.3, 

 June 14. 



lonisation Potentials of Copper and Silver. 



In their book on " The Origin of Spectra " Foote 

 and Mohler assign ionisation potentials of 7*692 and 

 7*542 to copper and silver. These are calculated 

 from spectroscopic data. I have recently succeeded 

 in obtaining low voltage arcs in the vapours of these 

 two metals. For copper, a voltage of 7-8 was found, 

 agreeing with the value given above as closely as 

 one would expect from observations on a low voltage 

 arc. For silver vapour, however, the value found 

 and verified by many observations was 6*0 volts. 

 There were indications of a resonance potential at 

 about 3*1 volts. 



This work is being continued especially into the 

 spectroscopic region. A. G. Shenstone, 



Physical Laboratory, 



University of Toronto, 



