160 



METEOROLOGY. 



Mtleorolo- 

 By- 



Observa- 

 tions on 

 tempera- 

 ture and 

 vegetation 

 by Mr. 

 Play fair. 



In a paper inserted in the Transactions of the Royal 

 Society of Edinburgh for 1800, Mr. Play fair pro- 

 posed, instead of striking the average temperature once 

 a month, as is usually done, to divide the whole year 

 into decads, or portions of 10 days, and to find the 

 mean of each. Twenty-one of these decads, viz. from 

 the 20th of March to the 20th of October, he calls the 

 vegetating season, and supposes that it is on the mean 

 temperature of that season, that the quantity of the 

 crop in a great measure depends. He assumes 40 as 

 the lowest temperature at which corn will vegetate, 

 and considers 56 as the mean temperature of a .good 

 vegetating season, so that the range between the mere 

 germination of vegetables, and the fullest maturity to 

 which they can attain in this climate, is 16 degrees. 

 M r. Playfair gives the following as the result of his own 

 observations for three years. 



Confirmed 

 by other 

 observa- 

 tion*. 



Whether the quantity of the crop, he adds, " may 

 be expected to be proportional to the excess of the mean 

 temperature of the vegetating season above 40, so that 

 the crop of 1799 would be to the crop of 1798 as 2 to 

 3, or in a greater or less ratio, may deserve to be more 

 accurately considered. There is reason however to 

 think that the variations of the crop, at least corn, will 

 be greater than in proportion to the variations of tem- 

 perature ; for if the mean heat of the vegetating season 

 were to fall as much below that of 1799) as the mean 

 of 1799 did below that of 1798, it would be reduced 

 to 46, a temperature so low as would certainly prevent 

 the ripening of corn altogether. By doubling the de- 

 ficiency of the heat therefore, we do a great deal more 

 than double the deficiency of the crop, so that the lat- 

 ter varies in a higher ratio than the former. The limit 

 at which corn will not ripen is probably higher than 

 46, and may perhaps be stated at 48.'' 



In illustration of Mr. Playfair's theory, we shall state 

 the mean temperatures of the seasons from 1815 to 

 1818, with the corresponding prices of meal, according 

 to the fiars of the county where the observations were 

 made. These prices cannot be supposed to be exactly 

 in the inverse ratio of the quantities of grain, but still 

 they will afford a general idea of the relative qualities 

 of the crops. 



Many of our readers will probably recollect, that a 

 considerable proportion of the corn crop in this coun- 

 try, in 1 8 16, did not arrive at full maturity, and that its 

 inferiority to that of the preceding year was much 

 greater than the difference in the temperatures of the 

 vegetating seasons. In both these respects therefore, 

 Mr. Playfair's conjectures are fully verified, and we 

 have no doubt that a more extensive collection of ob- 

 servations on the principle which he has recommended, 

 taking care to extend or shorten the length of the ve- 



getating season according to the latitude, would throw Meteorolo- 

 much light on the interesting subject of climate ; but 67- 

 we must leave this to be more fully discussed under the """">" 

 article PHYSICAL Geography 



As we have already found that observations of the Barometri. 

 thermometer at 1 morning and evening, give nearly cal bser- 

 the true mean temperature, there is reason to conclude vatloBS - 

 that they are as likely as any other hours, to give the 

 mean height of the barometer and hygrometer, when 

 these instruments are not constructed so a* to register 

 the extreme points to which they may rise or fall in 

 the absence of the observer. Self-registering barome- 

 ters are easily constructed, and have sometimes been 

 used, but as from the increased friction they are not 

 susceptible of great accuracy, we do not know that 

 more satisfactory results are to be expected from em. 

 ploying them. It is difficult indeed to find two baro- 

 meters of one kind, even by the best makers and on the 

 most approved principles, that will exactly coincide, 

 nor is it easy in any case, to avoid errors in adjusting 

 the instrument for observation. From this circum- 

 stance, as well as from the irregularity formerly com- 

 plained of in the times of observing, barometrical re- Often inac- 

 gisters are perhaps still more imperfect than those curate, 

 of the thermometer. Before any comparison can be 

 instituted between two such registers, it must be pre- 

 viously ascertained that the mercury employed in 

 botli i barometers is of the same specific gravity, that 

 the mode of adjustment, or contrivance for bringing 

 the surface of the mercury in the basin to the com- 

 mencement of the scale, is the same in each, and 

 that the observations are made precisely at the same 

 hours. These points being determined, it is still far- 

 ther necessary to make allowance for difference of tem- 

 perature and elevation, by both of which, especially the 

 latter, the length of the mercurial column is consider- 

 ably affected. Such of our readers as are conversant 

 with the use of the barometer, must be aware of the 

 difficulty of avoiding all these sources of fallacy, in re- 

 gistering the indications of that instrument. We have 

 found from repeated experiments, which have been con- 

 firmed by those of an eminent artist, that in bringing 

 the surface of the mercury in the basin to the point 

 from which the scale commences, the adjustment should 

 always be made in the same direction, and that it should 

 not be elevated in one observation, and depressed in 

 another. In our observations, whatever may be the 

 state of the instrument, we uniformly bring the mer- 

 cury below the true point, so that the final adjustment 

 is always made by elevating the mercury in the cistern. 



In order to exclude irregularities arising from dif- Corrections 

 ference of temperature, every barometrical observation to be made 

 might be reduced to what it would be, were the tern- in barome- 

 perature of the mercurial column at 32<>. This might tncal b " 

 be done very simply, and in most cases with sufficient 8e 

 accuracy, by allowing 3-thousandths of an inch for 

 every degree that the temperature differs from 32, 

 and either subtracting that quantity from, or adding it 

 to the observed height, according as the temperature 

 is above or below 32. The rule, strictly speaking, ap- 

 plies only when the observed height is 29-5, but in 

 the ordinary range of the barometer, the error is not 

 very considerable. 



A similar correction might be made, and with great 

 facility, for the difference of elevation between the po- 

 sitions of different barometers. It has already been re- 

 marked, that at moderate heights, the mercurial co- 

 lumn sinks one tenth of an inch for every 90 feet of 

 perpendicular ascent. Every barometrical observation 

 therefore may be reduced to what it would be at the 

 level of the sea, by allowing 1-thousandth of an inch 



