Bpias^" 
103 U 
ON THE CONSTRUCTION AND USES OF HYGROMETERS. 
ON THE CONSTRUCTION AND USES OF HYGROMETERS. 
By E. J. LOWE, Esq., F.R.A.S., &c. 
¥ARIOUS have been the instruments which philosophers have invented for ascertaining the hygro- 
metrieal state of the atmosphere. Catgut, wood, the beard of the -wild oat, &c, •were formerly 
used, but their hygrometrieal properties vanished by constant exposure. De Saussure used the human 
hail-, and l)e Luc a thin piece of transverse grain of whalebone ; but where variation in length, volume, 
or weight is the mode of action, no dependence can be placed upon the instrument. Daniell's 
hygrometer, which is a very good one, has many disadvantages — requiring very careful manipulation, 
and ether of the best quality. There is no instrument so good as a 
pair of good thermometers, the bulb of one being enclosed in thin 
muslin, to which is attached a piece of lamp cotton wick communi- 
cating with a vessel of rain water ; these should be renewed once a 
month. 
The conversion of liquids into elastic fluids is evaporation, and 
this goes on at all temperatures, from the hottest day in summer to the 
coldest night in winter ; and if we only call thirty inches the annual 
amount evaporated from the surface of the waters of this globe (a 
quantity in all probability much below the average), water to the 
amount of 62,000 solid miles is each year changed into clouds, which 
clouds condense in their turn and form the copious showers which 
water the plants and supply the springs. When we consider the moist 
condition of the soil, an additional supply of evaporation must be added 
to make up the deficiency in this calculation. The process is carried 
on in the following manner : — The great oceans supply the air with 
moisture ; the heat of the sun causing rapid evaporation, the aqueous 
particles arc carried into the atmosphere in an invisible state until 
they approach a cold current, when they condense, and are visible as 
clouds, which float along until a further condensation precipitates them 
in the condition of rain, snow, or hail ; these, when they fall on dry 
land, hasten along, accumulating rapidly, until the drops of rain form 
a stream, streamlets accumulate into rivers, and, at last, the drop is 
conveyed back to its parent, the ocean. The rivers all flow into 
the sea, carrying large volumes of water, yet the seas are never too 
full ; and why ? The oceans give back an equivalent, for evaporation is 
the cause, the making, of rivers, and is constantly going on to keep 
them flowing. Springs of water owe their supply to 
the same cause, and arc cold or warm according to 
the temperature of the different underground strata 
through flinch they happen to flow. A mountain 
stream, I. e., one originating on a high hill, is cold, 
whereas one whose reservoir is deep in the earth is 
warm; as for instance, the warm springs of llaih. 
imv and wet bulb tii MMcijiKii .u. or the still warmer ones of Iceland. Vapour is most 
plentiful at the equator, becoming gradually less abundant as wc near either pole: but in tropical 
countries it is held in an invisible state, whereas, in colder regions it is condensed into clouds. 
The dry and wet bulb thermometer is a modification of Dr. Mason's hygrometer. The latter has 
glass fountain for the supply of water, which is easily broken by frost, whilst the common drj and 
wet ball thermometer is furnished with a zinc cup, having a lid with a bole in the centre for the cotton 
conductor to pass through; the cup is situated away from the dry ball, and its surface covered, in 
order that the evaporation from the water may not influence its readings. The bulb, which is covered 
by muslin, and moistened by means of the water conveyed by capillary art ion up the cut tun wick, will 
show a temperature depending upon the following circumstances:—" The air in contact with the wet 
bulb gives enough of heat to vaporize t lie water, which being converted into vapour suffioienl in 
quantity to saturate the space which the air occupies, the reduction of temperature will he according to 
the quantity of heat, which has been combined in order to change its state from water to vapour." 
The difference bdicatedbythedryandby the wet bulb thermometer is occasionallj (between April and 
September), 18°, and frequently from 9° to 12° s in other mouths, seldom more than !>'. and more fre 
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