1850.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



345 



ON DEDUCTIONS FROM METEOROLOGICAL 

 OBSERVATIONS. 



On Deductions from Metrnrolngkal Ohseri-atinns. By John- Drew, 

 Esq., F.R.A.S., Member of the Council of the British Meteorolo- 

 gical Society. 



{Concluded from page 312.) 



Mean Temperature. — Dmrnal liiwffe. 



The mean temperature of a month or year has heretofore been 

 considererl the arithmetical mean of the hig-hest and lowest read- 

 ings of the thermometer recorded durins: that ]ieriod; thus it was 

 thought, that if the maximum and minimum readings were added 

 together, and divided by the number of obser\ations taken during 

 the time, the result would be the mean temperature for the period 

 through which the register extended. ^Ir. (Jlashier has shown, 

 however, that this estimate would be too high by a ([uantity vary- 

 ing with the month during which the observations were taken. 



At the Royal Observatory, Greenwich, meteorological observa- 

 tions have been registered since 1840, every two hours, tln-oiigliout 

 the day and night. If the times of observation are taken as 

 abscissse, and the temperatures be projected as ordinates, that is to 

 say, if a sheet of paper be divided into 24 parts, each represent- 

 ing an hour, and at each point perpendiculars be erected which 

 shall be proportioned to the temperature of each hour, the line 

 joining the upper extremities of these perpeiuliculars will repre- 

 sent the variation of the temperature during the day; the mean 

 temperature will be represented by a straight line, which will cut 

 off with the curve equal areas above and below. 



In like manner, if the mean temperature of each month be pro- 

 jected, the mean teni}>erature of the year will be represented by a 

 line which shall satisfy the same conditions. 



In the Philosophical Transactions, Part I. 1848, may be found a 

 paper by Mr. Glashier, "On the Corrections to be applied to the 

 Alonthly .^ieans of Meteorological Observations, taken at any 

 Hour, to Convert them into iMean jMonthly Values. " In this he 

 has shown, that to the mean of the observations taken during a 

 month at any hour, a certain correction must be applied to deduce 

 the mean value for the month. By a careful comparison of fiv'^ 

 years' observations, he has been enabled to tabulate these quanti- 

 ties, so that henceforth by applying them, the mean monthly \alue 

 may be deduced from observations taken at any one hour during 

 the day. Since the publication of these results, he has extended 

 his comparison through a period of eighty years, during which 

 observations on the temperature have been made at the apartments 

 of the Royal Society, Somerset House, all of which he has taken 

 the trouble to reduce; and he has found the same law of diurnal 

 variation to hold good. The application of his corrections gives 

 true results from the ( )xford observations, and seem ajqilicable to 

 all places inland; but whether exactly the same quantities will suit 

 every place in England, especially those on the coast, admits of a 

 doubt; they certainly must be altered for Dublin, and for this rea- 

 son, that the maxima and minima of meteorological jdienomena do 

 not occur at the same hour of the day, as at Greenwich; and the 

 apex of the curve projected as above, denoting the highest tempe- 

 rature, is much more pointed; in other words, the greatest heat 

 during the day is attained more suddenly, and declines more 

 rapidly at Dublin than at the Royal Observatory at Greenwich. 

 At the latter place, twice during the day, the mean temperature of 

 the air is at its mean value, and these times are as follows in the 

 several months: 



In January 

 In February 

 In March 

 In April 

 In May 

 In June 

 In Juiy 

 In August 

 In September 

 In October 

 la November 

 In December 



h. m. 



at 10 a.m. 



at 9 30 a.m. 



at 9 10 a.m. 



at S 40 a.m. 



at 8 25 a.m. 



at y a.m. 



at 8 a.m. 



at 8 20 a.m. 



at 8 5.5 a.m. 



at 9 a.m. 



at 9 20 a.m. 



at 10 a.m. 



and again 

 and again 

 and again 

 and again 

 and again 

 and again 

 and again 

 and again 

 and again 

 and again 

 and again 

 and again 



h. m. 



at 8 p.m. 



at (! 40 p.m. 



at 7 20 p.m. 



at 7 p.m. 



at 7 .'10 p.m. 



at 8 p.m. 



at 8 .'i p.m. 



at 7 20 p.m. 



at 7 20 p.m. 



at 7 p.m. 



at ii 4.5 p.m. 



at 7 2ti p.m. 



At these periods the temperature changes very rajiidly, or it 

 might be advisable to take observations at the times above specified; 

 but unless this were done with great precision as regards the local 

 time, a small error in that particular would introduce a large one 

 in the observations: it i.s, therefore, recommended, that the times 

 of observation should be those least liable to interruption, and that 

 they should correspond most nearly with those at wliich least 

 changes are taking place in the elements, which times the tables 

 will show. 



If the mean of the daily registrations of the maximum and 

 minimum thermometers be taken, the following quantities must be 

 subtracted to obtain the mean temperature for the month: — 



A\'e have thus, then, two entirely independent methods of deter- 

 mining the mean temperature, which mutually check each other. 

 One, from the mean of tlie Iiighest and lowest readings of the 

 tbermometer minus the above quantities; the other, by applying 

 .Mr. Glaisher's corrections to the observations taken at any hour 

 of the day. The first publication of the Biitisb Meteorological 

 Society is' a reprint of these tallies, witli the addition of another, 

 showing that the amount of cloud is also subject to certain laws, 

 and that the obscurity of the sky has its maxima and minima and 

 mean amount as well as the temperature of the air. Not that 

 these are the only variable elements in meteorological inquiry 

 which obey definite laws; the mercury in the barometer fluctuates 

 daily above and below the mean pressure, which may be ascertained 

 by the application of the tabulated corrections. Four times daily 

 the reading of the barometer is at its mean value: these times in 

 the several months are as follows: — 



By the application of the quantities in the tables, all of which 

 have been deduced from observation, the following phenomena may 

 also be reduced to their mean values: — 



I. The mean depression of the temperature of evaporation 

 below that of the air at the height of 4 feet above the soil at every 

 hour in each month. 



'2. The mean depression of the temperature of the dew point 

 below that of the air. 



3. The corrections to be applied to the monthly mean elastic 

 force of vapour, to deduce the true mean elastic force of vapour 

 for the month from the observations taken at that hour. 



4. For the mean quantity of aqueous vapour in a cubic foot 

 of air. 



5. For the mean degree of humidity. 



(!. To the weight of vapour in a cubic foot of air. 



I trust that the explanations I have attempted will show that 

 some progress has been made in the study of Meteorology, and 

 will convince observers that their records may be of service in the 

 cause of science, that they will excite an interest in inquiring 

 minds, and direct their energies in a useful channel. It is incum- 

 bent on the engineer not to neglect the science which may assist 

 him in supplying the increased demand for one of the necessaries 

 of civilised life. The medical practitioner — the recognised guardian 

 of the public health — the mariner, on whom rests the resjionsibility 

 of preserving life and property in crossing the ocean, and whose 

 experience has taught Idm the necessity of marking atmospheric 

 changes, cannot, with safety, disregard the science of Meteorology. 

 The British Meteorological Society is intended to form a depot for 

 the valuable observations of the officers of the navy and mercantile 

 marine; and that society will, I apprehend, soon enter upon some 

 systematic arrangement for the purpose of gaining over such intel- 

 ligent and competent allies. The agriculturalist is deeply interested 

 in our progress, in order that his skill may be exercised in develop- 

 ing those natural productions which are best suited to the climate 

 of the country he is called to cultivate. On this latter point I may 

 appropriately quote the words of a late writer in the Royal 

 Agricultural Society's journal, who has taken up the subject with, 

 great ability in an article 'On the Climate of the British Islands 

 in its Effects on Cultivation.' 



"C)f what avail, then, it may be asked, is the knowledge of such 

 a subject? That we may bend to tlie power we cannot control, 

 and learn to adapt our culture to the capabilities of the climate' 

 indeed, climate is the ruling principle of agriculture — the law 

 ■which governs the productions of different countries; and he who 

 yields the most enlightened obedience obtains the largest reward." 



46 



