154 



METEOROLOGY. 



Meteorolo- 

 gy- 



Method 

 proposed 

 l>y Dr. 

 UrcH-stcr 

 the most 

 accurate. 



Meteorolo- 

 gical obser- 

 vations in 

 general not 

 to he de- 

 pended on. 



Cannot be 

 compared 

 with one 

 another. 



doing so. This remark must be understood as apply- 

 ing only to the thermometers that have hitherto been 

 employed, for it is possible that a method may yet be 

 discovered of rinding the true mean temperature of any 

 given interval, with the greatest precision. Such a 

 method indeed has actually been proposed by Dr. 

 Brewster, by means of an instrument which he deno- 

 minates an ATMOSPHERICAL CLOCK, and of which a 

 short account will be found under that article. It was 

 again noticed under the article BAROMETER, to which, 

 as well as to the hygrometer, the principle is equally 

 applicable. From the friction that must necessarily 

 take place in such an instrument, the extremes might not 

 perhaps be recorded with the same accuracy as by a de- 

 licate self-registering thermometer ; but there seems to 

 be no reason to doubt, that it would give the average 

 of all the changes of temperature, pressure and moisture, 

 with the greatest accuracy. As the Atmospherical 

 Clock, however, has not yet been generally used, 

 we must still inquire how the problem in question 

 may be most accurately solved by the instruments 

 described in the preceding part of this article. The 

 only question therefore at present is, at what hours of the 

 dayandnight ought the observations of the thermometer 

 to be made, so as to give the nearest possible approxima- 

 tion to the mean temperature of the whole twenty-four 

 hours. 



For a considerable time after the thermometer came 

 into general use, little attention seems to have been 

 paid to the hours of observation, nor are they very ri- 

 gidly adhered to, even by meteorologists of the pre- 

 sent clav. It is no uncommon thing to see in registers 

 the state of the thermometer, morning and evening, 

 without specifying at what hour of either the observa- 

 tion was made ; or when the time is mentioned, to find 

 it an hour earlier or later one day than another. Such 

 registers may serve to gratify the curiosity of the indi- 

 viduals who keep them, but it is obvious they can add 

 little to our stock of meteorological knowledge. Their 

 results may accidentally approach to the mean tem- 

 peratures of the places where they are respectively ob- 

 served, but we have no certainty of their doing so, and 

 in as far as they have hitherto been employed as data 

 on which to found any theory whatever, they are 

 worse than useless. 



But the s^me objections are in part applicable, we 

 suspect, to not a few even of those registers in which 

 the observations have been uniformly made at certain 

 hours of the day. In some we find the hours of ob- 

 servation six in the morning and noon, in others eight 

 in the morning and two in the afternoon, in others 

 nine in the morning and three in the afternoon, ac- 

 cording to the convenience or particular opinions of 

 the observers. In short, we seldom find two registers 

 kept at the same hours, nor are the number of obser- 

 vations always the same. By some, one is considered 

 as sufficient, provided it be made at the hour when, 

 according to their theory, the temperature of the day 

 is about its mean, while others observe two, three, or 

 even four times during the twenty-four hours. Now, 

 without pretending to decide on the merits of the va- 

 rious hours of observation, or to determine which of 

 them are most likely to give the nearest approximation 

 to the true mean temperature, it must be obvious to 

 every body, who is in the least degree acquainted with 

 the fluctuations that are perpetually taking place in 

 the temperature of the atmosphere, that such registers 

 can admit of no comparison with one another. They 

 jnay afford a tolerably accura'e view of the relative 



characters of different years, at the respective places of Mcteoro] 

 observatioi\ ; but unless it can be shown that their re- gy. 

 suits approach all equally near to the true mean, they ^""Y"""' 

 furnish no assistance that can be relied on, towards the 

 solution of the important meteorological question, What 

 is the difference between the mean temperatures of 

 two places on the surface of the globe ? 



It might naturally have been expected, that after the Importance 

 invention of the self-registering thermometer, these of self-re- 

 differences in meteorological registers would have dis- gistering 

 appeared, and that whatever opinions individuals might lhcrmo ' u e- 

 entertain, as to the best hours for observing the actual *" 

 state of the thermometer, they would at least record 

 the maximum and minimum temperatures of every 

 twenty-four hours. This, indeed, has in many cases 

 been done, and has already contributed not a little to 

 give a precision and accuracy to the subject which it 

 did not before possess. Still, however, the use of 

 these instruments has been more limited, nnd the re- 

 sults of their indications less attended to, than could 

 have been wished. There seems to be a propensity 

 too, among certain philosophers, to undervalue these 

 results, arising perhaps from the difficulty which they 

 find in reconciling them with certain preconceived 

 notions and favourite theories of their own. It may . 

 be worth while therefore to inquire, upon what princi- 

 ple the mean of any two observations can be consider- 

 ed as a nearer approximation to the true mean of the 

 ' twenty-four hours, than that of the maximum and mi- 

 nimum of the same period. 



We cannot indeed pretend to demonstrate, that the Fallacy of 

 mean temjorature of any two or three given hours does a >>sumin 



not coincide with or approach very near to the true l ' 



i-.i 11 i i i ^ i . -. rature of 



mean of the whole day and night ; but it must be ob- any givcn 



vious, we think, that their agreement can be known hour as tht 

 only when the latter has been otherwise and accurately mean tcm- 

 determined. Suppose, for example, that the mean perature of 

 temperature at any t\vo hours, as eight in the morning lhe da ?- 

 and two in the afternoon, has been found by regular 

 observations continued for a series of years to be 50, 

 what grounds have we to conclude that 50 is the true 

 mean temperature of that place, unless it was previous- 

 ly known to be so, or without assuming as found, the 

 very point which the observations themselves are in- 

 tended to discover ? It is not enough that we can as- 

 sign plausible reasons of a theoretical nature, for sup- 

 posing that it must be either exactly or nearly the true 

 mean. The whole is a matter of experiment; and 

 though we may have determined with the utmost pro- 

 ciiion what is the mean temperature of any given hour 

 or hours, it still remains to be proved whether that 

 agrees or disagrees with the true mean temperature of 

 the day. We are disposed therefore to place very little 

 faith in many of the opinions that are advanced, and 

 the sweeping conclusions that are drawn, on this in- 

 teresting subject. We have been lately told, for ex- 

 ample, on the authority of Humboldt, that the mean, 

 or half the sum of the maximum and minimum observ- 

 ed during the day and night, or through the summer Sunset i 

 and winter, does not represent the true mean tempera- c mmen 

 ture and that between the parallels of 46 and 48, |j l um ' bolc 

 the thermometer at sunset indicates nearly the medium ,. ,u_ , 



.1-1 T 



temperature of the day. Now, in all this, it is obvious- for ascer . 

 ly taken for granted, that the true mean temperature uining i 

 had been accurately ascertained by means independent true uie 

 both of the maximum and minimum, and of the tem- temper; 

 perature at sunset ; and the question therefore still re- tur< 

 curs, how was it determined ? 



As we have not had an opportunity of examining 



