1188 



METEOROLOGY. 



[DBTKRMINATTOX OF HEIGHTS. 



any one to detect it, even in minute quantities, when it 

 occurs in the atmosphere in a free state. 



We have alluded, at page 315, to the presence or 

 absence of ozone in crowded localities, as a test of their 

 sanitary state ; and we may urge on our readers the 

 great advantage which will arise from an extension of 

 experiments of that kind. Wo there stated, that ozone 

 tests were variously affected, according to the proximity, or 

 otherwise, of crowded dwellings. The presence of animal 

 matter in the atmosphere seems to entirely prevent any 

 action on the test-papers ; and this would confirm the 

 idea that a chemical relationship, or affinity, subsists 

 between ozone and the carbon, hydrogen, or nitrogen 

 which gases, emitted from the animal body, contain. For 

 such purposes, the salts of manganese* are more delicate 

 as ozone tests, and are, therefore, to be preferred, in 

 many cases, to the iodide of potassium and starch papers. 

 In observing the effect of ozone on such tests, the depth 

 of colour produced, as compared with a standard, is to be 

 noticed : it is, therefore, desirable that specimen papers 

 should first be procured ; and they are sold at most of the 

 instrument-makers. Any quantity of the test-paper may 

 afterwards be easily made, by following the directions 

 given at page 11G4 ; and in using them, the direction of 

 the wind, whether it may have blown over a town before 

 being tested, or arrived in an unchanged and non-vitiated 

 state, with all other circumstances which might modify 

 the action of the free ozone, should be carefully noticed, 

 otherwise the observations would bo worse than useless, 

 and be only calculated to mislead. We have no doubt 

 that many curious particulars might be gathered by the 

 medical profession, in relation to endemic and epidemic 

 diaaMM^ if this interesting subject were more fully 

 understood. Such persons would have frequent oppor- 

 tunities, both in the localities and the sick room they 

 visit, of making valuable additions to the facts we already 

 I , 



BAROMETRIC DETERMINATION OP HEIGHTS. Rules for 

 determining the heights of mountains, <tc., by the baro- 

 meter, have been already given ;t which, for some com- 

 parative results, will suffice : we may, however, here 

 mention certain points which require careful attention, if 

 extreme accuracy be required. 



After a perusal of the pages devoted to a description 

 of the ascents of Mr. Glaisher, the reader will perceive 

 that the ordinary rule, of an equal decrement of tempe- 

 rature occurring at equally increased elevations, does not 

 always hold good; and therefore, that heights, 'deter- 

 mined on the assumption of the correctness of that rule, 

 must necessarily be erroneous. There are also other 

 causes of error, which must be taken into account, 

 and which were ably pointed out by Mr. Ball, in a paper 

 read before the Physical Section of the British Associa- 

 tion, in 1802. We shall avail ourselves of the report 

 given in the jtGututwn, for the purpose of laying the 

 leading facts before our readers. 



The author remarked, that serious errors are involved 

 in the ordinary process of reducing barometric observa- 

 tions, for hypsometrical purposes. This process involved 

 two assumptions ; first, that the volume of a column of 

 air, unequally heated, is nearly the same as that of an 

 equal weight of air of the same mean temperature ; 

 secondly, that the mean temperature of the column or 

 stratum of air, between the stations of observation, cor- 

 responds to the mean of the readings of thermometers 

 standing in the shade, at each station. The error in- 

 volved in the first assumption is not very considerable ; 

 that ariiiing from the second, is, on the contrary, highly 

 important M. Bravais was the first to propose a prac- 

 tical plan for applying a correction to the assumed 

 mean temperatures of the air, depending upon the hour 

 of the day, and the season of the year, at which observa- 

 tions are made ; but it is to M. Plantamour, the distin- 

 guished astronomer of Geneva, that we owe the fullest 

 investigation of this important subject. Having ascer- 

 tained, by careful levelling, the true height of the Groat 



8a* ont,, p. M4. 



. i ,::,. /'...,,-, I, 

 !8MM/,p.Ml. 



P. 771 ; *od Mittvnlon, pp. UJJ, U34. 



St. Bernard above Geneva, M. I'lnntninour finds that 

 the mean of all the barometric observations made during 

 eighteen years, deviates by fourteen feet, English, from 

 the true height ; and ho attributes this deviation, with 

 great apparent probability, to an abnormal depression of 

 the mean tem]>erature of Geneva, owing to the neigh- 

 bourhood of the lake. The readings of the barometer 

 and thermometer, at the observatories of Geneva and the 

 St. Bernard, are taken daily at nine hours or epochs. M. 

 Plantamour assumes that, on an average of a long period 

 of years, the moan of the observations taken at any one 

 epoch in the twenty-four hours, should give the true dif- 

 ference of height between the two stations, with an error 

 due to the difference between the mean of the readings 

 of the thermometer, at both stations, at the same epoch, 

 and the true mean temperature of the air in the inU-r- 

 Y<-iiiiig stratum. Calculating, then, the height of the St. 

 Bernard by the elements corresponding to each epoch of 

 the day, during the four summer months, from Juno to 

 September, he obtains a series of measures differing from 

 the true height ; those corresponding to the hottest hours 

 being in excess, and those appertaining to the coldest, 

 in defect of the true height. He then ascertains the 

 amount of correction, which being applied to the mean 

 sum of the readings of the thermometer, at each epoch 

 in each of those months, would bring out the true 

 height. In this manner he obtains a table, showing 

 what he calls the normal correction for each of the epochs 

 of the day, during the four summer months. 



There is good reason to believe, that in reducing baro- 

 metric observations, which are to be compared with those 

 of Geneva and the St. Bernard, the application of the 

 normal correction, ascertained in the manner above 

 stated, will, in general, give truer results than those 

 where it is not applied ; but as it is obvious that the 

 conditions of temperature, at the moment when a given 

 observation is made, are constantly varying from the 

 mean of the corresponding day and hour it follows 

 that a further supplemental correction should be made 

 on this account. To apply this further correction is a 

 matter of no small difficulty ; but the method employed 

 by M. Plantamour is as follows : Ho obtains from the 

 observations at Geneva and the St. Bernard, by inter- 

 polation when necessary, the elements corresponding to 

 the day and hour of the observation which is sought to be 

 reduced ; and from these he calculates the height of the 

 St. Bernard. The height so obtained, when comp;a< 1 

 with the measure, which is derived from the mean of the 

 readings of the same day and hour (as shown in his table 

 of normal corrections), furnishes a criterion by which to 

 judge of the conditions, with respect to temperature, of the 

 moment when the observations to be reduced were made. 

 M. Plantamour thinks it not difficult to iufor, from the 

 observations themselves, and from the general state of 

 the weather of the time, whether the moment was one 

 of atmospheric equilibrium, or the reverse. In the 

 latter case, the observation is treated as one of inferior 

 utility, to which a lower value should be assigned in the 

 final calculation. Supposing, on the contrary, the ob- 

 servations not to betray a disturbance of equilibrium 

 between the two stations, the deviation of the height, 

 as calculated for that particular moment, from the 

 height derived from the corresponding means, is the 

 measure of the amount, and sign of tlio supplemental 

 correction corresponding to the moment of observation. 



These facts acquaint us with a peculiar and local cause 

 of error, which adds another difliculty to the method of 

 measuring heights by means of the barometer. To some 

 extent the sources of error are analogous to those we 

 have already pointed out, when speaking (if the variety 

 of causes which affect a normal atmosphere in the 

 neighbourhood of, or immediately over, large towns ; but 

 they differ, in being more constant, and therefore more 

 eaaily detcrminable in their character. Many of our 

 readers may imagine that the method of finding the 

 elevation of accessible heights by the rules of trigo- 

 nometry,! would be sufficient for all purposes, ami render 

 us independent of the barometrical method ; but the 

 trigonometrical plan is not at all times possible; for, 



