307 



Reading of thermometer 40 deg., subtract -031. 



50 deg., -058. 



60 deg., -085. 



70 deg., -in. 



80 deg., -138. 



90 deg., -164. 



100 deg., -191. 



no deg., -217. 



Next add the correction for the height of the station above sea 

 level. This can be obtained exactly from Scott's tables, or roughly, 

 by taking - ioo inch for every 100 feet in height. If readings are 

 carefully taken and reduced, and the means computed every month, 

 it will probably be found that they oscillate on each side of 30 

 inches. In fact it may be taken as a first generalization that a 

 reading over 30 accompanies hne weather, and under 30 bad. But 

 it will be further noticed that the barometer is on the whole lower 

 in summer than in winter. Hence we may expect unsettled 

 weather w r ith a reading of 30*1 or 30*2 in winter, whereas it may be 

 fine when the barometer indicates 29*8 or 29-9 in summer. But we 

 must also pay attention to the rise and fall. The usual sequence is 

 as follows : The barometer commences to fall with fine, bright 

 weather, becoming warmer until the lowest point is nearly reached. 

 As this point approaches, the sky becomes overcast, and rain is 

 probable. After the minimum, as the barometer commences to 

 rise, the weather generally becomes squally, especially in winter, 

 with driving showers from the N.W. and W. These gradually 

 diminish in intensity, but may be expected occasionally as long as 

 the barometer commences to rise, or the wind remains in the west. 

 It not infrequently happens that the weather clears rapidly after 

 the minimum has been reached, but the barometer remains 

 stationary, or commences to fall a second time, and the wind backs 

 from W. or N.W. to N. or N.E. In this case prepare for even 

 heavier squalls within 24 hours, or probably less. The indications in 

 summer are not as a rule so well marked as in winter, but generally 

 when the barometer falls below 29^8 unsettled weather may be 

 looked for, and if in addition the mercury be very unsteady thunder- 

 storms may be expected. Do not mistake the diurnal change for a 

 falling barometer, the reading at 3 p.m. being almost always slightly 

 below that at 9 a.m. 



THERMOMETER AND HYGROMETER. 



The best form of hygrometer consists of two ordinary 

 thermometers mounted vertically side by side, one of which 

 has a strip of muslin attached to the bulb at one end and 

 dipping into a cup of water at the other. These thermometers 

 are known as dry bulb and wet bulb. Suitably exposed 

 the dry bulb indicates the " temperature in the shade," and the 

 two conjointly with the aid of Symons' hygrometrical tables, give 



