ON THE SEASONAL VARIATIONS OF TEJirEEATURE. 47 



Care mnst be taken never to turn the case upside down while the thermo- 

 meter is in it. 



4. Heading the Thermometer. — The paper scale inside the wide glass 

 tube, and behind the narrow tube containing the thread of mercury, is 

 divided into degrees ; the mark of each ten degrees is numbered from 10 

 to 140 ; and the mark of each five degrees is a little longer than the 

 others. The thermometer should be read to tenths of a degree, and the 

 reading put down as a decimal fraction. To read the thermometer, the 

 instrument must be held perpendicularly, with the top of the mercury 

 thread on a level with the eye. Suppose the top of the thread of mercury 

 to be exactly at the second degree mark above the long one numbered 

 40, then the temperature it records is to be written down as 42'0 ; should 

 the top of the mercury just barely appear above this mark, it is to be 

 written 42'1 (that is, 42 degrees and one-tenth) ; if decidedly above the 

 second mark above 40, and apparently not quite one- quarter of the way 

 to the third mark, it is 42-2 ; if scarcely beyond a quarter of the way, 

 42'3 ; if almost half way 42'4 ; if exactly half way between the two marks, 

 it is 42-5 (that is, 42 degrees and five-tenths, or one-half) ; if a little 

 beyond half way, it is 42'G ; if very nearly three-quarters of the Avay, 

 42*7; if a little over three-quarters, it is 42-8; and if almost up to the 

 third mark above 40, it is 42'9. The same way of reckoning applies to 

 the position of the mercury thread between any two degree marks. A 

 tenth of a degree is a very small amount, and the observer need not be 

 discouraged if at first he fails to read so closely ; if ho perseveres, by 

 cautious guessing he will soon become proficient. Mistakes are more 

 often made in reading the degree than the fraction ; for instance, one is 

 more apt to write 443 than 43"4 when the true reading is 43'3. Special 

 care must be taken to avoid this. 



5. Selection of Observing Station. — In a lake, an estuary, or the sea, the 

 observation should be made in a boat at some distance from the shore ; fail- 

 ing this, an observation may be made at the end of a pier or a steep rock, 

 provided the depth at low tide is at least three feet, and that the current 

 does not flow over rocks or ground which remain dry at low tide. 



In a river it is best to note the temperature in mid-channel from a 

 boat ; but in rapid streams an observation from the shore is equally satis- 

 factory. The best position is in a rapid run where the depth exceeds 

 three feet. Quiet pools and shallow shelving shores are to be avoided. 



It is important to describe minutely the nature of the river at the 

 point where observations are made, the direction in which it flows, the 

 height of the banks, the degree of exposure to the sun, and especially to 

 wind ; and a sketch map should be made in the observing book, to show 

 exactly at what point the thermometer is used. It is necessary to observe 

 always at the same place, which should be near the observer's house, or in 

 the line of his regular walks. 



6. Hozir of Ohservinff. — The temperature should be observed once 

 every day at the same hour. The Committee recommend 9 a.m., when 

 that is practicable ; or, failing this, 10 or 12. If convenient to the ob- 

 server, a second observation may be made in the afternoon or evening, 

 but at the same hour every day. 



7. Observing the Weather. — The date and precise hour of observation is 

 to be entered in the observing book. The column ' Remarks on State of 

 River and Weather ' is then to be filled up with as much accuracy as 

 possible. If there is a flood-gauge in the neighbourhood, its indication 



