164 



METEOROLOGY. 



Point of 

 deposition 

 coincides 

 with the 

 minimum 

 tempera- 

 ture nearly 



urged, and indeed will probably operate, against the 

 general use of the formula. This objection, however, 

 it is to be hoped, will soon be in a great measure re- 

 moved, by the labours of the ssme individual to whom 

 the scientific world is indebted for the original and pro- 

 found speculations of the. article alluded to. Mr. An- 

 derson proposes to construct a table, in which, from 

 the indications of the hygrometer, in any given state 

 of the thermometer ami barometer, the three facts stat- 

 ed above may be found by inspection. In this table, 

 we believe, the indications of the hygrometer are to be 

 expressed in degrees of Fahrenheit instead of the mil- 

 lesimal scale, and the calculations are to be adapted to 

 different heights of the barometer. In a table which 

 we have constructed from the same formula, we have 

 retained the millesimal division of the hygrometer, and 

 adapted the whole calculation to one state of the baro- 

 meter, viz. 29.610, which we found to be the mean 

 height at the place of observation. The results are 

 perhaps as near the truth as the nature of the subject 

 requires ; and at all events, the errors arising from the 

 fluctuations of the barometer cannot materially affect 

 the annual mean. They are so inconsiderable, indeed, 

 as scarcely to affect the accuracy even of a single ob- 

 servation, as is evident from the following example. 



Suppose that on two different occasions the hygro- 

 meter indicated 30 and the thermometer 50, but that 

 the barometer in the one case stood at 30 inches and 

 in the other at 29. By the formula, the quantity of 

 moisture in 100 cubic inches of the atmosphere, ex- 

 pressed in decimals of a grain, is .146', when the baro- 

 meter is at 30, and . 1 44 when it is at 29, being a dif- 

 ference of only .002, corresponding to a fluctuation of 

 one inch. A table, therefore, calculated for the mean 

 height of the barometer, will be sufficiently accurate 

 for all meteorological purposes. 



It was infered, from the principles investigated in 

 the article HYGROMETKY, that the mean quantity of 

 moisture in the atmosphere on any given day, will cor- 

 respond nearly with what would produce complete sa- 

 turation at the minimum temperature of that day ; and 

 this conclusion was verified by a reference to meteoro- 

 logical observations for 1815. From accidental cir- 

 cumstances, it cannot be expected that this coincidence 

 should take place every day, but it was found to hold 

 very nearly in the means of the whole year, and is far- 

 ther confirmed by the following observations for 1818. 

 The last 3 columns contain the mean results of 10 

 morning and evening. 



The difference between the point of deposition Meicorolo- 

 and the minimum temperature in the above table, 

 scarcely ever amounts to a degree, except in the p^J^ 

 months of March and April. In the latter it amounts ljon of 

 to 5 ; but this exception from the general rule may be anomalies 

 easily accounted for, from particular circumstances, in the 

 During that month the wind blew almost constantly above table. 

 from the north-east, so that a continual influx took 

 place of dry cold air from the northern regions. This 

 air having its temperature increased by 'approaching a 

 warmer climate, acquired also a greater capacity for dis- 

 solving moisture, and thus augmented the dryness of 

 the atmosphere beyond what is natural to this climate ; 

 while by the continuance of the wind from the same 

 quarter, a new wave, as it were, flowed in before the pre- 

 ceding one could receive any sensible augmentation of 

 humidity. A similar depression of the point of deposi- 

 tion below the minimum temperature, and arising from 

 the same cause, took place in April 1817. During the 

 present year (1819) the greatest depression, amounting 

 to 3.3, happened in the month of May, chiefly with 

 east winds. In general the difference does not amount 

 to a degree. 



We have not thought it necessary to detail at length 

 our observations with the hygrometer for the four years 

 preceding 1818. In general, the application of Mr. t i on8 . 

 Anderson's formula to these observations, gives results 

 corresponding nearly with those stated in the above 

 table. It would appear, therefore, that at the place 

 where the above experiments were made, the atmo- 

 sphere, in its mean state, at 10 morning and evening, 

 contains about 18 grains Troy in ten thousand cubic 

 inches, being 80 hundredths, or four-fifths of what 

 would produce complete saturation. A series of simi- 

 lar observations under other latitudes, and at different 

 elevations, is still, however, a desideratum in meteoro- 

 logy ; and till this be supplied, it would be premature 

 to pronounce with confidence on any tiling that has 

 yet been advanced, respecting the law of aqueous so- 

 lution in the atmosphere. At the same time, it is not 

 too much, we think, to hope that Mr. Anderson's re- 

 searches on this interesting branch of science, will, 

 when better known, lead to the discovery of some im- 

 portant facts on the formation of clouds, the production 

 of rain, and other atmospherical phenomena. 



The most satisfactory, and, at present, generally re- Button's 

 ceived theory, on the last mentioned subjects, (the for- theory of 

 mation of clouds and the production of rain,) is that rain - 

 proposed by Dr. James Hutton,and first published in the 

 Philosophical Transactions r>f Edinburgh in 1784. In 

 this theory the author supposes, what has since been 

 satisfactorily proved, that the dissolving power of air, 

 or its capacity for holding water in a state of vapour, 

 increases in a greater ratio than its temperature. From 

 the experiments of Saussure, and others who have suc- 

 ceeded him in hygrometrical researches, it appears that 

 while the temperature rises in arithmetical progression, 

 the dissolving power of the air increases nearly in geo- 

 metrical progression. This particular relation, however, 

 is not necessary to the truth of the theory in question, 

 which requires only that the humidity which the air is 

 able to contain, increase in a greater ratio than the tem- 

 perature. This being admitted, it follows that when Mixture 

 two portions of air, of different temperatures, and both of saturat- 

 containing as much humidity as they are capable of dis- ed airs at 

 solving at these temperatures, are mixed together, the different 

 joint quantities of moisture in the two portions will be tem P er 

 greater than the air, after intermixture, is able to duce"^^" 

 dissolve; and consequently a portion of the humi- position o 

 dity must be condersed or precipitated. This will moist uf 









