296 METEOROLOGY. 



occurred at a point witliin eight miles of the city, in May, 1854, is 

 represented to have been very violent. But we have experienced nothing 

 in this locality like that, proceeding from a dense nimbus, which sud- 

 denly arose from the southwest on the 13tli of May, 1855, and, while 

 discharging its watery contents, rivalled, in the vivid shocks, of its 

 well-charged battery, the thunder gusts of more tropical regions. 



The aurora borealis has been observed only once — on the night of 

 the 16th of December, 1857 ; the sky being entirely clear at the time, 

 the wind light, from the east — the thermometer reading 44°, and the 

 barometer 30.321 inches, reduced for temperature. This phenomenon 

 first appeared in a northeast direction, in the form of a diffused light 

 defined by an arch below. From this arch, of about 15° radii above 

 the horizon, the light extended in width apparently 10° above Alioth, 

 in the constellation of the Great Bear, and gradually spread over the 

 whole northern section of the heavens, the dominant hue being deep 

 rose. Its aspect, however, was frequently changed by the successive 

 appearance and shifting of streaks or columns of white light, which 

 seemed to be more conspicuous at either extremity of the arch. With 

 the exception of a somewhat similar phenomenon seen once at Sonora, 

 Tuolumne county, during the winter of 1852-'53, we have heard of 

 no other instance of the aurora being seen in California. 



Before proceeding to a consideration of the rains we would, in this 

 connexion, briefly refer to the transparency of the atmosphere for which 

 California has been noted. The relative frequency of clear and cloudy 

 days in summer and winter, as appears in the tables, although sub- 

 stantially correct, does not convey a just idea of the clearness of the 

 sky. The results are calculated from three daily observations; and if 

 it so happens that at either of these the least cloudiness is visible it 

 is recorded as a cloudy day, without regard to quantity. Now, one of 

 the peculiarities of the summer climate is, that if there be any cloud- 

 iness during the day, which is rarely the case, it is almost invariably 

 clear at night. Indeed, on this account, perhaps there is no region 

 better adapted to astronomical purposes ; for, as Sir David Brewster 

 expressed his wish^ " no clouds disturb the serenity of the firmament, 

 and no changes of temperature distract the emanations of the stars." 

 As to the quantity of cloudiness, this not having been estimated pre- 

 viously to the last two years, of course the results in this respect can- 

 not be regarded but as approximative to a constant, the number of 

 cloudy days having been in excess during 1856 and 1857. 



RIVER, RAINS, ETC. 



The rise and fall of the river at Sacramento is graduated by the 

 terms high and low- water mark, or zero. A solid column, surmounted 

 with a wind -vane, was set up by the city near the river bank in Sep- 

 tember, 1856, when the river had attained the lowest stage ever 

 known. The fig. 2 in the accompanying hydrographic scale agrees 

 with the zero in our published observations up to that date. The 

 mean depth of the channel of the river in this neighborhood is 16 feet 

 below low-water mark, and the width of the river is about 300 yards. 

 There is a tidal rise and fall of about one to two feet at Sacramento, 



