582 



NA TURE 



[April 21, 1892 



As regards the other matter referred to, Mr. Tittmann does 

 not mention the publication in which " Mr. Chaney states that 

 the standard air to which his result is reduced weighs 0'3077 

 grains per cubic inch." The only publication known to me is 

 Mr. Chaney's paper in the Proceedings of the Royal Society, 

 and it does not contain any such statement. 



I have always been taught to regard a standard weight as a 

 standard of mass, and therefore independent of such conditions 

 as temperature, pressure, and the material in the other scale 

 pan ; whereas, it appears that Mr. Chaney, by direction of the 

 Board of Trade, has made a determination which is only true 

 for a particular density of the surrounding air, and a particular 

 density of the weights in the other pan. 



For scientific purposes a standard of reference should be free 

 from variable elements, and should be of the utmost attainable 

 simplicity. For commercial purposes sleterminatiOns .to six | 

 figures are frivolous. 



Mr. Tittmann's reductions appear to contain two errors. 

 Instead of adding the weight of a cubic inch of air, he ought to 

 have added the difference between this and the weight of the air 

 displaced by the weights in the opposite pan. Again, he takes 

 the metre as 39 '3700 inches, whereas Clarke's value is 39*370432, 

 and Kater's 39*37079. 



I have had some correspondence with Mr. Chaney since the 

 publication of my new edition, and have had an erratum slip 

 printed, which I trust you will allow me to subjoin, as it may 

 be useful to several of your readers ^ J. D. Everett. 



5 Princess Garden?, Belfast, March 28^ 



Addenda and Corrigenda. 



Page 6t,. In reducing Cailletet's experiments, '0000026 

 should have been added instead of "0000039. 



Page 77. Add — Violle's determination of velocity of sound 

 is 331*10 ± o-i. Ann. de Chim. XIX. March, 1890. 



Page 176, line lo. For Wuilleumeier, 1890, r^a^/Wuilleumier, 

 1890, Lippmann method. 



At end of page 164, a^^^Expressing C in amperes, R in 

 ohms, and T in seconds, the heating effect in gramme-degrees 

 is C^RTI^-2 = ■2^C'RT. 



Page 35. Mr. Chaney's determination here quoted was not in- 

 tended as a determination of the density of water, but of the 

 apparent weight of water when weighed in air of density 

 •001 21684 against brass weights of density 8*143. The cor- 

 recting factor for deducing the weight in vacuo or true density is 

 I 'ooi 0687, which will change the value '998 752 obtained in 

 the text into -999 82, to compare with Tralles' "999 88. 



Mr. Chaney's result is for distilled water deprived of air, and 

 Tralles' appears to be for ordinary distilled water. According 

 to results recently obtained by the Vienna Standards Commis- 

 sion (IVied. Ann., 1891, Part 9, p. 171), water deprived of air 

 has the greater density, the difference being '0000032 at 0° C, 

 and '0000017 at 62° F. These differences are too small to affect 

 the above comparison. 



Influenza in America. 



In my copy of " Johnson's Dictionary of the English Language 

 in Miniature, to which are added an alphabetical account of the 

 heathen deities and a copious chronological table of remarkable 

 events, discoveries, and inventions, by the Rev. Joseph Hamilton, ! 

 M.A., second American edition, Boston, 1806" (i2mo, pp. 276), 

 I find on p. 275, " Influenza in North America, 1647, 1655, 

 1697-98, 1732, 1737, 1747, 1756-57, 1761, 1772, 1781, 1789-90, j 

 1802." I 



It is quite possible that these dates are well known, but they | 

 are new to me, and may be of interest in connection with the ; 

 recent epidemic. Edward S. Holden, 



Mount Hamilton, March 29. 



DC/ST COUNTING ON BEN NEVIS. 

 "\yyiTHIN the last itw years quite a new factor has 

 * * been introduced into the study of meteorology— 

 namely, that which treats of the dust particles in the 

 atmosphere, of the number of dust particles present in 

 the air at any time, and the efTect of dust in the air on 

 climate and weather changes. It is now beginning to be 

 recognized that the study of dust and its behaviour in the 

 air forms the stepping-stone to the study of almost all 

 NO. I I 73, VOL. 45] 



meteorological problems which deal with clouds and pre- 

 cipitation, solar and terrestrial radiation, and in a general 

 way with the diurnal and annual variations in the tem- 

 perature and pressure of the atmosphere. Mr. Aitken's 

 work in originating this branch of science, and in making 

 and discussing numerous observations of the number of 

 dust particles in the air of various places in this country, 

 as well as on the Continent, at various altitudes, is pretty 

 well known already (see Nature, vol. xli. p. 394). Mr. 

 Aitken's results and conclusions were looked upon as 

 being of such importance as to warrant some of our 

 leading meteorologists to apply to the Research Fund of 

 the Royal Society for a grant to enable them to equip 

 the Ben Nevis Observatory with Aitken's dust-counting 

 apparatus. The application was successful, and instru- 

 ments were at once ordered, and in due time erected at 

 the Observatory. 



The apparatus consists of two dust counters, one a 

 portable form for use in the open air, and the other a 

 laboratory form for use inside the Observatory. The 

 latter is fixed in the middle room of the tower, and has 

 pipes leading out to the free air, so that it is possible to 

 observe with it in almost all sorts of weather and at any 

 hour day or night. The principle on which these instru- 

 ments are constructed, so as to make the tiny invisible 

 particles of dust visible and easily countable, is pretty 

 well known already. Briefly it is this. To make the par- 

 ticles visible, the air containing them is saturated with 

 water vapour, and by a stroke of an air-pump it is there- 

 after cooled so much as to cause a condensation of the 

 vapour on the particles, whereby these are thus made 

 visible. Ordinary air is so dusty that if the receiver were 

 full of such air it would be impossible to count the par- 

 ticles, and to make them easily countable the following 

 process is resorted to. First, the chamber or receiver, 

 whose capacity is accurately known, is filled with pure 

 dustless air by means of an air-pump and filter. Then 

 a fifth, a tenth, or any other fractional part of the amount 

 of pure air inside is taken out, and the same amount 

 of dusty air allowed in. In this way the density of the 

 shower caused by condensation is completely under the 

 observer's control. A small graduated stage is placed 

 one centimetre from the top of the receiver, so that all 

 the dust above this falls on to it, and by means of a 

 magnifying glass all the particles on one or more of the 

 small squares of the stage are easily counted. Then, by 

 multiplying by the reciprocals of the various fractions 

 used we arrive at the number of dust particles in a unit 

 of the free dusty air. In making an observation, the mean 

 of ten such tests is taken as the number of particles 

 present for that time. 



Observations were begun at the Ben Nevis Observatory 

 with the portable instrument in February 1890, and with 

 the other instrument in the following summer. During 

 the whole of that year the work done was mainly pre- 

 liminary, as great difficulty was experienced in getting 

 the dust work to fit into the general routine of Observatory 

 work. The dust inquiry is not like some other special 

 inquiries, that can be prosecuted for a certain time, and 

 then discontinued after definite positive or negative 

 conclusions thereanent have been arrived at, but must, on 

 the other hand, be carried on side by side with the other 

 observations of meteorological phenomena, as pressure, 

 temperature, humidity, &c., with any of which it is of 

 equal importance, and having once been admitted into 

 the general routine of meteorological observations it 

 must be kept on. This fact was soon seen on Ben Nevis 

 from the extraordinary variations that were observed in 

 the dustiness of the air with changes of weather ; and it 

 was attempted to make continuous hourly observations of 

 the dust as of the other elements. It was found, however, 

 that this could not be done without crippling the general 

 routine, this being as much as the two observers at the 

 Observatory could well cope with. In February 1891 



