148 



METEOROLOGY. 



gy- 



Mcteorolo- but, like the expansions and contractions of the thermo- 

 meter in its rude state, before the temperatures of melt- 

 ting snow and boiling water were ascertained to be 

 fixed points, they are vague and indefinite quantities, 

 and it reulains therefore for science to supply this desi- 

 deratum by the Application of instruments, which, 

 while they detect the operation of the same causes, may 

 shew likewise the amount of that operation. Still, 

 however, there is nothing unphilosophical in supposing 

 that there may be many substances, both in the ani- 

 mal and vegetable kingdom, more readily affected by 

 atmospherical changes, and consequently acting as me- 

 teorological instruments of greater delicacy than any 

 that philosophy has yet devised, or art constructed ; and 

 it can never, therefore, be derogatory to the honour of 

 science to avail itself of the aid of any class of pheno- 

 mena, even though they may have been long and fa- 

 miliarly known to the vulgar and illiterate. Meteoro- 

 logy has already been indebted to some of the most 

 common of these phenomena for the most valuable in- 

 struments that it now possesses ; and there is nothing 

 that forbids us to hope, that a more careful investiga- 

 tion of the above, and other similar facts, may not lead 

 to the discovery of more delicate and important instru- 

 ments than have yet been employed. 



Though, in the preceding sketch, we have purpose- 

 ly confined ourselves to the popular view of meteorolo- 

 gy, we are aware that this branch of our subject is far 

 from being exhausted ; but to have entered on a more 

 lengthened detail, however interesting, would have ex- 

 ceeded the limits prescribed to the whole of this article. 

 Our object, therefore, has simply been, to give our 

 readers a specimen of what meteorology was in its early 

 state, and of the progress which may be made in the 

 investigation of the subject, independent of the instru- 

 ments by which the researches of modern times have 

 been assisted, and to which we apprehend the attention 

 of philosophers has been too. exclusively confined. We 

 proceed now to consider the nature of these instru- 

 ments, and the discoveries to which they have led. 



Design of 

 the pre- 

 ceding 



ketch. 



Definition 

 of the at- 

 mosphere. 



Changes 

 which it 

 undergoes. 



Instru- 

 ments for 

 measuring 

 these 

 changes. 



Of Meteorological Instruments. 



The atmosphere, as defined in another part of our 

 work, is that invisible fluid which surrounds the earth 

 on all sides, and which has been found, by various ex- 

 periments, to be heavy, compressible, and elastic. For 

 an account of the chemical and physical properties of 

 this body, we refer to our articles ATMOSI'HERE, CHE- 

 MISTRY, PNEUMATICS, &c. our business at present be- 

 ing only to explain the methods which have been em- 

 ployed to ascertain the nature and extent of the changes 

 which it has been found to undergo. These changes 

 chiefly refer to its temperature, weight, moiilure, and elec- 

 tricity; and the instruments by which they are respec- 

 tively measured, are the thermometer, barometer, hygro- 

 meter, and electrometer. The result of these changes 

 is in some cases wind, and in others rain or snow, which 

 have also been subjected to measurement ; the intensity 

 of the former by the anemometer, and the quantity of 

 the latter by the ptiivimeter, or rain-gage. In describ- 

 ing the nature and application of these instruments, we 

 shall endeavour to render our account intelligible to the 

 general reader, from a conviction that a simple and po- 

 pular view of their principles and use, is still a deside- 

 ratum in works on meteorology. Many, we know, are 

 prevented from taking an interest in the subject, from 

 not understanding the nature or the application of the 

 instruments to be employed; and if we can therefore 



5 



in any degree contribute to the more extended use of 

 these instruments, and consequently to the multiplica- 

 tion of meteorological observations, we shall consi- 

 der our labours as of more service to the science in its 

 present state, than the most ingenious theory, or the 

 most profound speculations. 



TheThermometer is an instrument employed for mea- 

 suring the heat of bodies in general, and, among others, 

 that of the atmosphere. It is a well-known fact that 

 all bodies are expanded, or have their bulk increased by 

 heat, and are contracted or have their bulk diminished 

 by cold, that is, when heat is abstracted from them. 

 These variations of bulk are conceived to be propor- 

 tional to the variations of heat, and it is upon this prin- 

 ciple that the thermometer has been constructed. It 

 consists of a glass tube AB (Plate CCCLXX1V. Fig. 1.) 

 of a very small bore, having one extremity A blown in- 

 to a bulb. This bulb and part of the tube or stem is 

 filled with a liquid, generally mercury, which is found, 

 on various accounts, to be of all others best fitted for 

 the purpose. If the instrument in this state be applied 

 to a body warmer than itself, the mercury will be seen 

 to ascend in the tube ; because the expansion of the 

 glass being much less than that of the mercury, the 

 bulb is no longer capable of containing the enlarged 

 volume of the latter, which consequently rises into the 

 empty part of the tube. In like manner, if the instru- 

 ment be applied to a body colder than itself, the bulk 

 of the mercury in the bulb being diminished, that por- 

 tion of it which is in the tube will descend, to supply 

 the diminution of volume ; and this will be found to 

 take place even when the tube is held in a horizontal 

 position, the cohesion between the particles of mercury 

 being such, as to admit of no separation of one portion 

 of the mass from another. Hence it is obvious, that 

 every change of temperature, that is, every increase or 

 diminution of heat in the atmosphere, will produce, on 

 a thermometer exposed to it, a corresponding expansion 

 or contraction of the mercury ; and it only remains, 

 therefore, to find out some method of expressing the 

 amount of these variations in such a way, as that the 

 indications of one thermometer may be compared with 

 those of another. This is accomplished by means of a 

 graduated scale CD attached to the tube, the divisions 

 of which are thus determined. It has been found that 

 snow or ice in a melting state is always of the same 

 temperature, and that the heat of boiling water, under 

 the same pressure, is also uniform. These fixed points 

 being determined, by immersing the instrument first 

 in melting snow, and afterwards in boiling water, are 

 transferred to the scale, and the distance between them 

 is divided into a certain number of equal parts ; the di- 

 visions being carried downwards below the melting, or, 

 as it is improperly called, the freezing point, as far as 

 is thought necessary, or as the scale will admit. Hence 

 it is obvious, that two thermometers having the freez- 

 ing and boiling points thus determined, and the space 

 between them divided into the same number of equal 

 parts or degrees, must in similar circumstances uni- 

 formly indicate the same temperature. The divisions 

 themselves will be larger or smaller according to the 

 relative capacities of the bulbs and stems, but this cir- 

 cumstance does not in any degree affect their indica- 

 tions. Neither is it necessary that the space between 

 the freezing and boiling points should be divided into 

 one number of parts rather than another. This is al- 

 together arbitrary, and may consist of 50, 100, JOOO, 

 or any other that may be thought convenient. In com- 

 paring the indications of two thermometers, it is only re. 



Meteorolo- 

 gy- 



Of the ther. 

 momeler. 



PLATE 

 CCCLXXIV. 

 Fig. 1. 



Different 

 tbermome- 

 trical 

 scales. 



