182 ALLlSOi^ — ON THE METEOROLOGY OF HALIFAX. 



clear weather. Observations of corona for that same year give a 

 large proportion of succeeding wet days, but not as a necessary 

 sequence. ^ 



In 1871 — and I take years not immediately following, that the 

 series may be less partial, though necessarily brief — I counted 6 

 lular halos and 17 coronae. The former were exactly divided as to 

 their successors : three wet and three fine days. The latter were also 

 very nearly so, being followed by 9 wet days, and 8 dry. Sum- 

 ming up these 35 observations, we find the halos preceding wet and 

 dry days, in the proportion of 5 to 4, and the coronas giving a per- 

 centage of result closely similar. By a wet day I mean a day on 

 which precipitation is appreciable — or in other words measures .01 

 inch in the guage. If we include fog and mist, and all kinds of 

 inappreciable precipitation in our calculation, the resulting dry days 

 will dwindle to one third of the whole, after the two species of 

 moon rings. 



To separate truth from error, then, both halos and coronte 

 indicate moisture, and are therefore frequently followed by rain or 

 snow ; but not necessarily so, as the condition of the earth and the 

 different strata of atmosphere must be carefully considered, especi- 

 ally in connection with their capacity to hold moisture. 



The phenomenon variously known as hoar frost, white frost, 

 and rimy frost, is very widely considered to be a sure forerunner 

 of a chan2:e from fair weather to foul. There is an atom of truth to 

 induce this belief, but I think that it can be shown that the general 

 deduction from this small source is erroneous. Indeed this belief 

 is directly contradicted by another, viz., that a dew at night is 

 followed always by a dry day. Both of these suppositions cannot 

 be correct, for the difference between hoar frost and dew is merely 

 one of season. Dew may be called the summer hoar frost, and 

 this the winter devf. I need scarcely say here that terrestrial 

 radiation is the cause of these deposits. By the casting off of heat, 

 the earth's surface, as the sun's rays fall more and more obliquely, 

 becomes cooler, till the dew point is reached, and the moisture of 

 the warmer air becomes visible. Naturally, then, we should 

 expect this phenomenon to occur every night ; and it would do so 

 were all conditions always fulfilled ; but a clouded sky is a blanket 



