It is far from our intention, by these remarks, to un dervalue the researches of the experimental philoso pher, or to insinuate that the study of meteorology should be confined to the collection and arrangement of such phenomena as have been stated above. On the contrary, we admit, that these phenomena, of themselves, can afford no accurate information, either as to the na ture or extent of atmospherical changes. They may indicate these changes, but their indications are not susceptible of being correctly measured or definitely expressed. They are the result of certain influences exerted by the atmosphere in certain circumstances ; but, like the expansions and contractions of the thermo meter in its rude state, before the temperatures of melt ing snow and boiling water were ascertained to be fixed points, they are vague and indefinite quantities, and it remains therefore for science to supply this de sideratum by the application of instruments, which, while they detect the operation of the same causes, may slim likewise the anr.unt 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 (." 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 arid 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 rea ders 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 instruments, and the discoveries to which they have led.
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 riments, to be heavy, compressible, and elastic. Fur an account of the chemical and physical properties of this body, we refer to our articles ATmosru ERE, CHE mi sTRY, PsEu31Kries, &c. our business at present be ing only to explain the methods which have been em plo)ed to ascertain the nature and extent of the changes which it has been found to undergo. These changes chiefly refer to its temperature, weight, moisture, 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 or the former by the anemometer, and the quantity of the latter by the pluvimeter, 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 in any degree contribute to the more extended use of these instruments, and consequently to the multiplication of meteorological observations, we shall consider our labours as of more service to the science in its present state, than the most ingenious theory, or the most pro Ibund speculations.
The Thermometer 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 hulk 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 .1.1 "cilia' 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 CCCLXXIV. Fig. 1.) of a very small bore, having one extremity A blown to a bulb. This bulb and part of the tube or stem is tilled with a liquid, generally mercury, which is found, on various accounts, to he of all others best fitted for the purpose. If the instrument in this state he 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 pact of the tube. In like manner, if the ment be applied to a body colder than itself, the hulk of the mercury in the bulb being diminished, that tion or it which is in the tube will descend, to supply the diminution of volume ; and this will he 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, 1000, or any other that may be thought convenient. In com paring the indications of two thermometers, it is only requisite that the number in each be known, though it is certainly desirable that a uniform scale should be adopted.