110 The Weather. 



fiery hue at sunset to au apparently cloudless sky. Its 

 height is 15,000 or 20,000 feet above the sea. At a lower 

 and more advanced stage of the southerly current, Cirro- 

 cumulus is formed, and the well-known appearance of a 

 "mackerel-backed sky" is produced. Cirro-cumulus is the 

 immediate precursor of the wind, or of rainy weather, accord- 

 ing as its outlines appear sharp and oily, or the reverse. It 

 changes by insensible degrees into cumulo-stratus, whose 

 ragged masses sail across the sky, or settle into thick and rainy 

 weather according to the moisture of the air. Between cirro- 

 cumulus and cumulo-stratus, a quantity of fleecy clouds are 

 often found; and in the direct light of the sun, the vivid 

 hues of the mackerel's skin, as described by Sir John Her- 

 schel {Meteorology), are sometimes reproduced. It also forms 

 a corona of highly- coloured circles round the moon, especially 

 in unsettled conditions of the weather, when a south-west 

 current veers towards the north, and northerly winds are 

 felt along the ground. 



A cloud is called Nimbus, or rain-cloud (as its name implies), 

 when, from its lower surface, it precipitates rain, snow, or hail. 

 Local clouds of this kind are seen both in mountain- valleys and 

 in the plains, and are never absent when thunder and lightning 

 are produced. 



As it is not intended in these articles to enter into practical 

 details, the particular difficulties of rain-guage measurements 

 in connection with this subject, now attracting much attention, 

 must be passed over in silence, and the subject of rain, hail, 

 and snow will be deferred to a chapter on the winds. 



In the foregoing paragraphs, a knowledge of the construc- 

 tion and use of simple instruments only has been assumed — 

 such as the different forms of the thermometer. An acquaint- 

 ance with the standard form of barometer will, in the same 

 manner, be found a sufficient introduction to the subject of the 

 winds. Attention must, however, be paid to two principal 

 corrections of the barometer, without which it fails to indicate 

 the atmospheric pressure. The first of these corrections is that 

 for temperature of the mercury, and of the brass scale (in 

 perfect instruments), as shown by the " attached thermometer. " 

 The other is for the height of the instrument above the sea. 

 The height of the mercurial column, " corrected to the freezing 

 temperature of water, and reduced to the level of the sea/' 

 represents the pressure of the atmosphere at any place ; and 

 observations of this kind can only be consulted for comparison 

 with one another. For this purpose, Lowe's Barometrical 

 Tables are extremely useful, and more abundant materials are 

 contained in the Meteorological Tables, published by the Smith- 

 sonian Institute of America. 



