21] 



29. IjfSTEtrCTIONS FOR TAKING MeTEOKOLOGICAL OBSERVATIONS. 



By William Marriott, F.M.S., 



Assistant Secretary of the Meteorological Society. 



Communicated by J. Hopkinson, Hon. Sec. 

 [Read 12tli April, 1877.] 



As this Society will naturally take an interest in the science of 

 Meteorology, and as some of its members may make, or intend 

 making, observations with the view of determining the climate, etc., 

 of Hertfordshire, I have much pleasure in acceding to the request 

 of your Secretary to give a few instructions upon the best method 

 of taking meteorological observations. 



Meteorological observations to be of any scientific value must be 

 made on a uniform plan at all stations, otherwise the lesults will 

 not be comparable. The Meteorological Society is very stringent 

 in this matter of uniformity, and only accepts those observers who 

 will comply with its requirements, and whose stations and in- 

 struments are found to be satisfactory on inspection. 



Instruments. — The necessary instruments are : — Standard Baro- 

 meter ; Dry-bulb Thermometer ; Wet-bulb Thermometer ; Maxi- 

 mum Thermometer ; Minimum Thermometer ; Rain-gauge ; and 

 Stevenson's Thermometer Stand. It is also desirable to have 

 a Black-bulb Maximum Thermometer in vacuo; a Minimum Thermo- 

 meter for terrestrial radiation ; and an Anemometer. 



All the instruments should be verified at the Kew Observatory, 

 so that the corrections for index error may be ascertained. 



Barometer. — The Barometer may be either a Fortin or a Kew 

 Standard. It should be mounted in a room which is only subject 

 to very slight changes of temperature, must hang vertically, and 

 be in a good light. 



When the mercury in the tube falls, that in the cistern rises in a 

 corresponding proportion, and vice versd, and unless this variation 

 be taken into account, the readings will not be correct. In the 

 Fortin barometer this difiiculty is overcome by having an adjust- 

 able cistern, so that the mercury can always be maintained at a 

 constant level by being brought into contact with a fixed ivory 

 point. In the Kew barometer the cistern is closed, but the error 

 arising from the change of level in the cistern (technically termed 

 "the error of capacity") is overcome by contracting the divisions 

 on the scale, so that the inches are somewhat less than true inches. 

 The height of the mercury is determined by bringing the bottom of 

 the vernier to form a tangent to the top of the column, and then 

 reading off the divisions on the scale and vernier. 



All barometers have a thermometer attached to them, with the 

 bulb inserted in the brass case, so as to be between it and the glass 

 tube ; this is supposed to show the same temperature as the mercury 

 in the barometer. By its means we are able to determine the 

 proper correction to be applied to the reading of the barometer to 



