5H 



NATURE 



[April i, 1880 



earth's surface, and presumably at great elevations also, supports 

 the generally-received conclusion that the pressure variation is 

 an effect of the annual inequality of temperature. 



Having thus good p-imd facie evidence for believing that by 

 far the greater part of the annual variation of pressure may be 

 explained on simple hydrostatic principles, I thought it desirable 

 to test this conclusion by Mr. Archibald's method of substraci ion, 

 making use of somewhat fuller data than were at his disposal 

 when he wrote the letter above referred to. The observations 

 I have adopted are those of Roorkee, 887 feet above the sea- 

 level ; Dehra, 2,232 feet; Chakrata, 7,052 feet, and Leh, 

 11,503 feet elevation. The first three stations lie within a few 

 miles of each other, their latitudes being 29 52', 30 20' and 

 30 40' N. respectively. Leh is at a considerable distance to 

 the north, in latitude 34 10' N. The four stations are situated 

 neatly 111 the same meridian, the difference of longitude between 

 the most westerly and the most easterly amounting to less than 

 half a degree. 



The mean annual values of temperature and pressure at these 

 four places are the following : — 



PRE351T E. 



( 12 years) 28-889 inches. 

 (12 „ ) 27-567 „ 

 ( 4 „ ) 23-225 ,, 

 ( 4-6.. ) «9'°59 » 

 With the exception of the temperature figures for the winter 

 months at Leh, the data are all for sufficiently long periods to 

 be taken as fairly representing normal values of temperature and 

 pressure. From these the average temperatures and barometric 

 weights of three successive strata of air have been calculated, 

 and the results, together with the variations in each month from 

 the annual average values, are given in the next table. 



From these figures it is evident that when the temperature is 

 above the average the pressure is below it, and vice versa. The 

 only exceptions to this rule, which applies to each separate 

 stratum of air as well as to the whole thickness of 11,616 feet, 

 occur in the months of April and October, when the variations 

 of the barometric pressure from the mean of the year are within 

 the limits of the probable error of the observations. 



The variations of the density of each layer of the atmosphere 

 are also very nearly proportional to the temperature variations, as 

 they would be if the air expanded and contracted freely with 

 changes of temperature. Thus the mean decrease of density 

 fo,- one degree of rise in temperature between Roorkee and 

 Chakrata is '00235. At tne mean temperature of these two 

 stations, 65-6°, the co-efficient of expansion per degree Fahr. 

 is '0019. The observed variation of density is thus slightly 

 greater than that which would be caused by change of tempera- 

 ture alone, but the difference may be completely accounted for 

 by the larger proportion of aqueous vapour in the air in the hot 

 than in the cold months. 



Taking the mean pressure of the lowest stratum of air (that 

 between Roorkee and Dehra) to be the arithmetical mean of the 

 pressures observed at the top and bottom, and supposing the 

 mean tension of vapour in it to be similarly obtained, we may 

 calculate the ratio of its densities in the hottest and coldest 

 months by the usual formula :— 



J_ _ P - jf 460 + t 

 d' P' - If: 460 + C 

 With the data P = 28-428, F' = 27-982,/= -301,/, = "695, 

 and the temperatures given in the preceding table, the ratio of 

 the density in June to that in January comes out "921, while the 

 ratio of the barometric weights is '916. A similar calculation 

 for the stratum between Dehra and Chakrata gives the ratio of 

 the densities in the hottest and coldest months equal to -933, 

 that of the barometric weights being -943. 



It follows from these results that the annual variation of the 

 barometer over the plains of India and up to a considerable 

 elevation in the Himalayas may be explained by simple hydro- 

 static principles. A moment's consideration will also show that 

 the double oscillation observed at the hill stations, which is 

 romewhet puzzling at first sight, may be explained in the same 

 way, without bringing in any hypothetical saturated antimonsoon 

 current. 



It is the combination of this, at first sight, anomalous variation 

 in the upper regions of the atmosphere, with the variations due 

 to simple changes of density below, that gives rise to those 

 peculiarities of the annual change of pressure in India which 

 led Mr. Brown to give the weight of his great name in meteor- 

 ology to an opinion that is clearly erronecus. 



Allahabad, 18th February S. A. Hill 



Gunnery Experiments 



I have read with interest the leading article on Gunnery 

 Experiments in Nature, vol. xxi. p. 437. The question seems 

 to me to be one not alone of build, but — and perhaps rrinci pally 

 — of muzzle-loading versus breech-loading, and of rifling for or 

 without studs. The Admiralty seem to think so, as appears, I 

 presume, from their resolution to adopt breech-loading for the 

 turrets of the Colossus. With breech-loading double loading is 

 an impossibility, as well as jamming of studs, since there are 

 none, at least in the first artilleries of Europe. I dare say Sir W. 

 Falisser's build is better than any other known in England; but 

 then with it the best guns would be breech-loaders. 



Contrary to the grand practice of Europe, England has hitherto, 

 with characteristic tenacity, retained muzzle-loading for great 

 guns. Now she will, I apprehend, have to refoi m and to pay 

 enormous suns as a penalty, besides enduring the very incon- 

 venient feeling of temporary inferiority in a means of great 

 importance. X. 



The Hague, March 15 



A Museum Conference 



I DEPRECATE as strongly, though not so violently, as "Aca- 

 demicus," an association to talk about museums, but I cannot 

 agree with his reasoning on the subject of museums and their 

 curators. I have had tw enty years' daily experience of museum 

 work, and at the risk of being dubbed a pretentious curator I 

 can assert I have brought an average intelligence to bear on my 

 work. With a certain amount of sjmpathy for the strictures of 

 "Academicus" on the multiplication of conferences, I am yet 

 free to assert that in no department of public work might and 

 could greater public advantage result from close association of 

 officials than from a union of museum curators. A provincial 

 curator must often be oppressed with the conviction that he is 

 spending weeks over a task which is already, in some other 

 locality, done to his hands, and he must likewise know that the 

 labour he is in other instances performing, and the objects 

 he is manipulating would be sufficient for the wants of a dozen 

 institutions like his own. He knows that he wants what others 

 have, and that from his abundance others might be filled. Then 

 again, in a general museum, the presiding officer, to be tho- 

 roughly efficient, should be master of the circle of the sciences, 

 and have a familiar acquaintance with all arts and art. But 

 science is all-embracing, art is long, and the arts of to-day are 

 obsolete to-morrow. I say in contradiction of "Academicus" 

 that museum officials only know their business when they know 

 their ignorance, and that proper salaries are not their only or 

 chief want. In a scientific sense the best men would be the 

 worst museum curators, and were the munici) alities of Great 

 Britain each to offer the salary of a cabinet minister for the ser- 

 vices of a museum superintendent, I do not think the institutions 

 would thereby at once be so much revolutionised as " Acade- 

 micus " thinks. 



I am happy to be able to announce that the Council of the 



