206 PHYSICS. 
variable, and are always accompanied by thunder and lightning, or electrical 
phenomena. Around them calms generally prevail. They carry along 
with them all objects taken up. They arise sometimes from the clouds, 
sometimes from the water; their mass consists of watery vapor, and not 
of solid water; their greatest diameter varies from 2 to 200, their greatest 
height from 30 to 3000 feet. A figure of a water-spout is seen in 
pl. 27, fig. 1. Land whirls, according to Horner, have the same origin 
with water-spouts ; they are, however, more impetuous and destructive, 
not being retarded or impeded by the weight of water. Peltier has found 
recorded 116 whirls, of which sixty belonged to the land, and fifty-six to the 
sea. Of the former the oldest was in 1456, of the latter in 1664. Of the 
entire number twenty-nine (eighteen by land and nineteen by sea) exhibited 
a circular motion, and twenty-two (thirteen by land and nine by sea) no 
internal motion at all; forty-one (twenty-five by land and sixteen by sea) 
were accompanied by thunder and lightning ; ten carried the objects taken 
up in a direction opposite to the wind ; sixteen ended with hail ; six vanished 
in a cloudless atmosphere without doing any damage. Of the water-spouts, 
three shook fresh water over the ship, although on the sea; in fifteen the 
water was seen to rise; in eight to sink down; two connected an upper 
cloud with a lower ; in eight cases a sulphureous smell was perceived; in 
six cases several spouts were seen at the same time; thirty-four exhibited 
peculiar phenomena. 
5. Of the Moisture of the Atmosphere. 
A greater or less quantity of water is always dissolved in the air in the 
form of vapor. This vapor, under ordinary circumstances, is invisible, but 
becomes visible when it returns to its liquid state in the form of dew or 
frost, fog or cloud, rain or snow. Certain instruments, called hygrometers, 
are employed to determine the amount of watery vapor contained in the air. 
The hygrometers formerly employed had for their basis some organic or 
inorganic body, which became elongated or increased in weight: by 
absorbing watery vapor, the amount of absorption being in proportion to the 
amount of vapor in the atmosphere. The best instrument of this kind was 
the hair hygrometer of Saussure. A soft pale human hair is boiled for half 
an hour in a solution of sulphate of soda, and then for a few minutes in pure 
water, washed off in cold water, and dried in the shade. It is then fastened 
by its upper extremity to a little tongue, the other end being wound round a 
minute pulley provided with two grooves. Round the other groove passes a 
silk thread, from which is suspended a weight of several grains to keep the 
hair in a state of constant tension. An index, fastened to the pulley, 
traverses a graduated arc whenever the pulley is turned in either direction 
by the contraction or expansion of the hair. To graduate the instrument and 
fit it for its proper office, it is first introduced into a receiver, in the bottom 
of which has been placed heated chloride of calcium or concentrated 
sulphuric acid, and after the air has been exhausted, the place noted where 
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