KDouiledge & Selentif je flems 



A MOXriILN' JOTRXAL OF SCir.NCE. 



Conducted by MAJOR B. BADEN-POWELL and E. S. GREW, M.A. 



Vol. I. No. 



[NEW SERIES] 



AUGUST, 1904. 



C Entered at n 

 Stationers' Hall. J 



SIXPENCE. 



Contents and Notices. — See Page VII. 



ProLcticaLl Meteorology. 



I. "TerT\pera.tvire of the Air. 



By ^^'II.I.IAM Marriott, F.K.Mct.Soc. 



It is curious what vajjue ideas many people have respect- 

 ing the weather, and of the instrimients eniployetl for 

 recording its changes. The word " ghiss " is often used 

 indiscriminately for the Barometer and for the Thermo- 

 meter ; and the word " Barometer" is also occasionally 

 used to indicate the Thermometer, and vice versa. It is not 

 supposed that any of the readers of " Knowledge " have 

 fallen into these errors, but perhaps some information on 

 meteorological instruments and the results derived from 

 their observation may be of service. 



We are all affected in some way or other by weather 

 changes, and our feelings tell us when it is cold or warm ; 

 but for systematic observation of these changes it is 

 necessary to have a definite standard for comparison, and 

 for this purpose we use the Thermometer. 



It goes without saying that everj'one knows that a 

 thermometer consists of a fine glass tube with a bulb 

 blown on at one end, and that it is partly filled with some 

 liquid, usually mercury or alcohol, which expands on 

 being heated, and contracts on bemg cooled. The tube 

 is marked off in degrees so that the changes of the 

 liquid can be measured on a definite scale. The Fahren- 

 heit scale is the one used m this country, in which the 

 freezing point is 32", and the boiling point 21?. ". On the 

 Continent the Centigrade scale is generally employed, in 

 which the freezing point is o", and the boiling point 100'. 



For meteorological purposes the thermometers should 

 be sensitive instruments and of the best construction, and 

 they should be verified at the Kew Observatory in order 

 that their errors may be determined and the necessary 

 corrections supplied. In order to obtain the highest and 

 lowest temperatures self-registering thermometers must 

 be used. The maximum thermometer may be either on 

 Negretti and Zambra's or on Phillips's principle. In the 

 former the tube is bent near the bulb, and the bore 

 greatly contracted ; the mercury, in expanding, is forced 

 through this contraction but is not permitted to recede 

 into the bulb on a lowering of temperature. It therefore 

 remains at the highest temperature. In Phillips's ther- 

 mometer a small air bubble divides the mercurial column, 

 the detached portion remaining at the extreme position 

 to which it has advanced, thus registering the highest 

 temperature. 



In the minimum thermometer spirit is employed, and 



in it theiL- is immersed an index. When the temperature 

 falls the spirit draws the index along with it, but on 

 rising again the spirit passes the index, leaving it at the 

 lowest point to which it iiad been drawn. 



In order that these instruments may indicate as nearly 

 as possible tiie true temperature of the air, and that the 

 results at various places may be comparable with each 

 other, they are placed in a .Stevenson screen, with their 

 bulbs 4 feet above the ground (Fig. i.) This screen, 

 which is a louvre boarded box painted white, is placed 

 in the open over grass, and away from walls, i.S;c. 



In this country it is the recognized custom to read the 

 thermometers at <) a.m. and to enter the reading of the 



Fiif. 



-Stevenson Thermometer Screen. 



maximum thermometer to tlje previous day. For all 

 practical purposes the mean temperature for the day may 

 be obtained by adding the maximum and minimum 

 readings together and dividing the result by 2, thus : 

 Max. 65-0^ min. 43'o" = mean 54'o°. From these two 

 thermometers we are thus able to obtain for each day the 

 highest, the lowest, the mean, and the range of tempera- 

 ture. They do not, however, indicate at what times the 

 extremes took place, so it is the custom at first-class 

 observatories to employ photographic self-recording in- 

 struments for this purpose. Previous to the introduction 

 of photography it was the practice at the Royal Observa-' 

 tory, Greenwich, from 1840 to 1847, for the observers to 

 read the thermometers every two hours, day and night 

 which was a very laborious proceeding. At the observa- 

 tory on Ben Nevis hourly observations are regulirly 

 made, as it is impracticable to employ self-recording 

 instruments in that extremely damp and cold climate. 



