It is very fortunate for the science of meteorolo gy that the simple mercurial thermometer has be come, under the present mode of manufacture, per haps the most satisfactory, complete, and simple of any of our philosophical apparatus. We admit the carelessness with which it is often made, and the want of agreement of instruments of the same construction which sometimes occurs; but we as sert that this want of agreement has been over stated, and that the very backward state of this, the most elementary thermometric branch of me teorology, and the slow and doubtful steps which it even yet makes, is owing but very trivially to this cause, and almost entirely rests with the observers themselves. Most persons who observe the ther mometer with even more than ordinary attention, arc quite ignorant of the real difficulties in the practical use of the instrument, and how their la bours may contribute best to the advancement of the science they wish to promote. In the most meagre register, the choice of hours, the accuracy to which they are kept, the position of the instru ment, are all subjects of considerable difficulty, and requiring- perseverance and practical knowledge; but should inquiries with the instrument be pushed beyond so meagre a diary, the consideration of time, and place, and affecting circumstances, whe ther remote or direct, rise to paramount impor tance, and if not duly attended to, destroy every value of such observations. Under the manifold difficulties of the subject, and the want of extensive and systematic co-operation under which so valu able an instrument as the simple thermometer. and the light which it throws on the state of the atmo sphere, have so long laboured, we are disposed to think the most important foundation-stone ever laid for the science of meteorology, to be the series of hourly observations of the thermometer made at Leith Fort, under the direction of the Royal Soci ety of Edinburgh, and at the instance of their late secretary Dr. Brewster, which was continued with unremitting vigilance for four years, and the results of the two first years of which have been published by Dr. Brewster, in a most important paper in the Edinburgh Transactions, Vol. X. The light there thrown upon the daily thermometric curve, renders it, we think, very well fitted for a brief notice in this place, being, as we have no hes itation in saying, far the most scientific tribute which has been paid to the value of the instrument. The horary thermometric curve for 1824 and 1825, is represented in Plate DXXV, Fig. 11. and it will easily be seen how nearly it approaches the form of a parabola. That it really does so, is shown by the following table, where the difference never exceeds a quarter of a degree of Fahrenheit. In this paper, too, a complete solution is given of one of the greatest problems in the application of the thermometer to meteorology, the proper hour of observation; the mean tempera ture occurred in 1824 at 9 h. 13 m. A. M. and 8 h. 26 m. P. al.; in 1825 at 9 11. 13 M. A. N. and 8 h. 28 M. P. m., a most extraordinary coincidence, which sets the value of these tables in a true light, demonstrates the real accuracy attainable in the subject, and proves the value of two years of con sistent and systematic observation, above the undi gested mass of heterogeneous materials collected by the labours of single observers in a century. Dr. Brewster has given tables for reducing the observa tions for any hour of the day, or pair of hours, to the true mean, for which we must refer the reader to the paper itself, glad, however, of having had an opportunity of introducing to his attention so im portant a subject.
We propose now to give some hints for the ap plication of the thermometer to practical meteor ology, a subject, as we have just observed, most palpably neglected, and the importance of which has been so overlooked, that, as far as we know, nothing like a systematic set of experiments have been published on the influence of disturbing causes in the determination of atmospheric temperatures, nor has any attempt been made to classify these sources of error. In some of our most practical
works, all directions on the subject are concentrated in such meagre terms, as "to avoid all kinds of radiation;" but unless observers have attended to the nature of these, the causes by which they arc produced, and the extent of the error to which they may give rise, such general cautions are wholly vain. We have but to look at the absurd statements given in the meteorological contributions of ephe meral journalists, of preternaturally high tempera tures alleged to have occurred in remarkable sea sons; even the temperature of 100° and upwards, have often been stated as occurring in this country in the shade. We may cite the very extraordinary summer of 1826, as a striking example of this, and we may refer to a paper, on that occasion, in the Edinburgh Journal of Science for October of that year, as a specimen of the nice precautions requi site in ascertaining with accuracy such extraordi nary temperatures. When the temperature of the air remains for any tine above 80°, no attention can be too great to avoid errors of radiation and reflection. The great importance of the subject, the neglect it has received, and the absolute neces sity of attending to it, to do the thermometer any justice as a meteorological instrument, induces us, before concluding this article, to give a bare out line of the subject, as one deserving of much atten tion in filling up.
The author of the present article has devoted some time and care to this inquiry, and particu larly in the summer of 1828, made at his leisure some experiments connected with it, but without any immediate view of publication, and conse quently without such systematic arrangements, and such a provision of apparatus as he would other wise have adopted. During the present summer (1829) he intended to have continued these upon an enlarged and improved plan, but the extraordinary changeableness of the season, and an almost unre mitting course of cloudy weather, deprived him of his best opportunities. He is therefore compelled to draw for illustration of his ideas upon his older sources of private information, of which, though upon an imperfect scale, he vouches for the accu racy, and they will aid him in pointing out•at least what he conceives to be the track in which such an inquiry should be pursued.
The deviations of the thermometric results from the real temperature, are either from internal or external causes. The former arise from the inapt ness of any index of expansion we can use to as sume the identical temperature of the air at any moment. Upon these we mean not at present to dwell; they have already been partly discussed in a former part of this article, when alluding to the variable capacities of bodies for heat. Alcohol ther mometers we have shown to he more sluggish in i their impressions than mercurial ones, and air is more rapidly affected than either. Hence the most essential error of the Sympicsometer, an instru ment already alluded to : its whole accuracy de pending on the perfect coincidence of the indica tions of the gaseous and mercurial bulbs, a certainty of which in many cases it is very difficult to attain.' In observations where the change of temperature is rapid or considerable, the sluggishness of alcohol thermometers may sometimes be sensible ; but in most cases it is not so, as will be shown by the ac companying table, in which case, however, circum stances were rather favourable, the wind being in an advantageous 'quarter for promoting an equili brium, the exposure of the window, close to the sill of which both instruments were suspended, be ing N.N.W.