METEOROLOGY. 



153 



Mtteorolo- objection ; but there is another which renders it equal- 

 ity- ly unsatisfactory at all seasons of the year. Though a 

 *"~~(~^ r portion of air, as we have already remarked, is im- 

 bibed by the humid surface of the ball, and rises into 

 the part of the tube from which the water has subsided, 

 it is in so small a quantity as to exert very little pres- 

 sure on the surface of the water. The consequence is, 

 that during a shower, the rain that falls on the surface 

 of the ball is forced into the interior, so that the wa- 

 ter again rises in the tube ; and though the elasticity 

 of the air above must at length counteract this process, 

 it is still sufficient to render the indications of the in- 

 strument altogether uncertain. AM a proof of this, we 

 shall mention one instance among several others that 

 might be stated. On an evening of a dry summer day, 

 the water in the tube of the atmometer stood at 100'. 

 During the night there was a heavy fall of rain, and 

 next morning the surface of the liquid was as high as 

 15. This fact, we apprehend, is decisive as to the 

 merits of the instr nnent. Mr. Leslie has shewn in his 

 treatise on the relations of air to heat and moisture, 

 that it may be successfully applied to various important 

 purposes in the arts, and in certain circumstances to 

 meteorological researches ; but it is totally unfit for 

 measuring the quantity of evaporation during an inter- 

 val of any considerable length. 



i Since writing the above, we have received from the 

 friend to whose hygromttrical researches we shall fre- 

 quently have occasion to refer in the course of this ar- 

 ticle, a description of an Atmometer which he has late- 

 ly invented, .ml which is undoubtedly the simplest as 

 well as the most ingenious instrument of the kind 

 hitherto proposed. It conMt of a bent'glass tube, 

 DEF, (Fig. 8.) of sufficient width to admit of 

 liquid moving i.i-.ly from one pan to another, and 

 swelling out into the bulbs BC and : .> this 



tube at A is introduced a quantity of alcohol, which, 

 after being conveyed into the bulb or wider tube EF, 

 is thrown into a state of ebullition, and while the steam 

 is issuing from A, the tube is then ,lly sealed, 



so that the air is completely expelled from the space 

 ABCDE. The bulb BC U then covered with mois- 

 tened silk or paper, and the instrument freely exposed. 

 In consequence of the pressure of the air bring re- 

 moved from the surface of the alcohol in the bul' 

 portion of that liquid passes into vapour, and occu- 

 pies the empty part of the lube. Were the whole of 

 the instrument at the same temperature, this process 

 indeed would quickly be stopt by the pressure 

 vapour itself on the surface of the alcohol ; but as the 

 bulb BC has its temperature reduced by the external 

 evaporation from the moistened silk or pjper, the va- 

 pour which rise* from EF is there condensed, and rum 

 down in a liquid state into the tube AH. This distil- 

 lation goes on more or less rapidly, according to the 

 degree of cold induced upon the bulb BC ; that is, in 

 proportion to the external evaporation ; and conse- 

 quently, the quantity of liquid collected in the tube 

 AB is a measure of that evaporation. When the atmo- 

 sphere is complnjv saturated with moisture, or when 

 the evaporation erases, the temperature of AB will be 

 the same as that of any other part of the tube, and the 

 di'tillatic-n therefore, for the reason already stated, will 



Graduation The measure of evaporation thus found, is expressed 

 f the icale. J n inches and decimals of an inch, by means of an at- 

 tached scale GH. the divj.ions of which are determin- 

 ed by experiment. Suppose, for example, that the 

 instrument i.i exposed in similar circumstances with an 

 VOL. xtv, PART i. 



evaporating basin, and that the quantity evaporated Mcteorolo- 

 from the Utter in a given time, as determined either S3- 

 by weight or measurement, is found to be one-tenth of "* *"Y" 

 an inch, while the alcohol distilled by the former in 

 the same time, fills the tube AB to the depth of one 

 inch ; then the scale being divided into inches and 

 tenths, will indicate tenths and hundredtbs of an inch 

 of evaporation. By increasing the proportion between 

 the diameters of EF and AB, the quantity of evapora- 

 tion may be measured to any degree of minuteness re- 

 quired. In using the instrument, the tube EF is to 

 be sheltered from rain by enclosing it in a case or co- 

 ver, to prevent its temperature being reduced below 

 that of the atmosphere by subsequent evaporation, and 

 the bulb BC is to be kept constantly moist by means 

 of a small cup containing water attached to the tube 

 immediately below it, the silk or paper being in con- 

 tact with the water, or from an adjoining vessel, as in the 

 case of the hygrometer. The instrument is placed in a 

 vertical position, and is prepared for a new observa- 

 tion by inverting it, so that the distilled alcohol may 

 be conveyed back to the tube EF. It is to be hoped 

 that this beautiful and ingenious contrivance, will soon 

 meet with that reception among meteorologists-, to 

 which its merits so well entitle it. The instrument has 

 already been constructed, and is lound to possess the 

 utmost delicacy. It is probable that it may, in time, 

 supersede the use even of the hygrometer. 



Application of Meteorological Instrument*. 



In considering the application of meteorological in- 

 struments, and the discoveries to which they have K il, 

 we shall follow the name order tlut we ha.c nh-i-rved 

 in explaining their nature and j>r IK i|.K--. Tin- fn-t 

 object of inquiry, indeed, anil one M Inch of all others 

 has engaged the attention of philo Ciphers, is tl 

 thud of determining the average or mean temperature 

 of the atmosphere at any given place. The solution 

 of this problem is sufficiently inteiesting in itself, but 

 it has become till more so from the other important 

 questions which it involves. It is intimately connect- 

 ed with the ile-. .11 of the law that regulates the 

 ill tribution of hest over different climates, and it is 

 only by accurately ascertaining the mean temperature 

 of a place at different and distant periods, that v. 

 ever hope to solve the question which has been so 

 much agitated of late, whether the temperature of this 

 country, or of the globe in general, is increasing, sta- 

 tionary, or diminishing. We shall then-fore consider 

 the subject at as great length as our limits will per- 

 mit. 



The most accurate method of ascertaining the mean 

 temperature of any place, is to note the thermc 

 at the end of every indefinitely j-hort interval, add all 

 the temperatures together, and divide the amount by 

 the number of observations. The quotient will giv'c 

 the average or mean temperature for the day, month, 

 year, or any other period during which the observa- 

 tions wer continued, and the shorter the intervals, the 

 more accurate will be the result. This method, how- 

 ever, being for obvious reasons impracticable, various 

 expedients have been proposed for approximating to the 

 mean by a less laborious induction of facts. Indeed, 

 the necessity of limiting the number of observations to 

 two or three in the course of twenty-four hours, has 

 been ahiH-i universally admitted ; for however desir- 

 able it might be to have them taken more frequently, 

 few arc found to possets either leisure or patience for 

 u 



To deter- 

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the atmo- 

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meteorolo- 

 87- 



Difficulty 

 of solving 

 it accurate- 



