214 W. MARRIOTT — METEOROLOGICAL OBSERVATIONS. 



conducting; thread should be washed with warm water, so that 

 every particle of ice may be melted off, otherwise the thermometer 

 will give too low a reading. 



The thermometers must be mounted in a Stevenson's screen, 

 which has double louvres all round, allowing a free current of 

 air to pass through, and preventing the sun shining on the 

 thermometers. The screen should be placed over grass in a freely 

 exposed situation ; it is desirable that it should never be in the 

 shade, and it must not be placed within ten feet of any wall. It 

 must be mounted on four posts, so that the bulbs of the dry- and 

 wet-thermometers shall be four feet above the ground ; and the 

 door should open to the north. The thermometers should be placed 

 as near the centre of the stand as possible. A convenient arrange- 

 ment is to mount the maximum and minimum on two small uprights 

 in front of the dry and wet, in such a way that the scales of the 

 latter can be seen between the two former. 



The observations are made as follows : — Having let down the 

 door of the screen, the dry- and wet-bulb thermometers are to be 

 read first, so that they may not be affected by the nearness of the 

 observer. The minimum thermometer is to be read next by 

 noting the position of the end of the index farthest from the bulb. 

 (The end of the column of spirit shows the temperature at the time 

 of observation.) The maximum thermometer is read last by 

 noting the degree at which the end of the column of mercury is 

 lying. When this has been done, the instruments should be 

 looked at again to see that no mistake has been made in reading 

 them ; the maximum and minimum should then be set in the 

 manner ahready explained, and the door closed. 



Spirit thermometers are liable to a serious fault, viz. the 

 evaporation of the spirit from the column, and its condensation at 

 the top of the tube. We sometimes see in the newspapers accounts 

 of extremely low temperatures having been registered at certain 

 places, but they are very often traceable to this fault, for the ther- 

 mometers may have as much as 5° or 10° of spirit lodged at the top 

 of the tube. As it is not always possible to see the spiiit at the 

 top, it is very desirable to occasionally place all the thermometers 

 in a basin of water and compare them together, for we can then 

 see if the thermometers are working properly or have gone wrong. 



The instrument used for measuring solar radiation is what is 

 called the "Black-bulb Thermometer in vacuo." It consists of a 

 sensitive maximum tlu!rmomet(!r, which has its bulb and one inch 

 of the stem coated with dull lamp-black; this thernu»noter is 

 inclosed in a glass jacket, from which the air is completely ex- 

 hausted. It should be mounted on a post four feet above the 

 ground, with the bulb directed to the S.E. The diiiorence 

 between the maximum in the sun and ia the shade is the amount 

 of solar radiation. 



Foi- deteriiiiuing the intensity of terrestrial radiation a sensitive 

 spiiit minimum thermometer must be used, and placed upon short 

 grass. 



