METEOROLOGICAL OBSERVATORIES. 153 



be expected, variations of the deepest thermometer are much less than 

 those of the thermometer nearest the surface, and also follow the sur- 

 face variations by a much longer interval. The depth of the longest 

 thermometer is not sufficient to give any valuable results as to the 

 increase of the temperature of the earth with depth; it is the compari- 

 son of the changes of the three thermometers with those of the air 

 thermometer that is the valuable point. 



We then approach the two stands, or screens, carrying thermome- 

 ters employed for taking the temperature of the air, and we look with 

 some respect on the central dry bulb thermometer — which is the stand- 

 ard thermometer for Greenwich temperature. It is placed with the 

 standard wet bulb on the older form of open stand set up in the time 

 of Sir George Airy and Mr. James Glaisher, and used in its present 

 form, with some slight modifications, ever since the commencement of 

 observations in the year 1841. It is commonly known as the " Glaisher 

 stand," and it is so contrived that it can be turned round on its verti- 

 cal axis, and so always be kept with its back to the sun, to secure a 

 proper shade for the instruments. The other screen, set up in the year 

 1887, aud which is of the form familiar to most of you, is known as the 

 "Stevenson screen," and is of the pattern now used by the observers 

 of our society. Both screens carry, in addition to the dry and wet 

 bulb thermometers, ordinary self-registering maximum and minimum 

 thermometers for eye observation. 



We come to the shed under which is placed the apparatus for photo- 

 graphic registration of the dry and wet bnlb thermometers first set up 

 in the year 1848. Here, in a light-tight box, are the two thermometers 

 so ingeniously arranged that their indications are continuously photo- 

 graphed on a sheet of sensitive paper fixed to an upright drum, which 

 is slowly carried round by clockwork, so as to bring successively fresh 

 surfaces under the beam of light which passes through the clear glass 

 of each thermometer tube, while it is of course impeded by the opaque 

 columns of mercury, so that when the images are duly brought out 

 there appear on the sensitive paper two broad traces, each bounded 

 below by a horizontal wavy line corresponding to the height of the 

 mercury in the two tubes. 



The register of the wet bulb stands immediately below that of the 

 dry bulb. The light is automatically interrupted for a short time at 

 every hour, producing on the developed sheet thin white columns, each 

 for some definite hour, so that any change of temperature may be 

 known, as well as the exact time at which it occurs. The readings of 

 these thermometers are reduced by comparison with those of the 

 standard dry and wet bulbs on the Glaisher or revolving stand. 



If there were time, many interesting points could be mentioned, such 

 as the extremely rapid fall of temperature that will at times take place 

 on the occasion of sudden changes of wind, notably in squalls. Again, 

 in frost, it is interesting to remark how, as the temperature passes 



