218 PHYSICS. 
bent tube, C, with a glass tube, D, from which latter the height of the water 
in B can always be ascertained. If A and B have the same cross-section, 
the height of the water in B will express the depth to which the rain water 
would stand on the earth in a given time, provided that no evaporation 
and absorption took place. The rain gauge must be set in an open place, 
so that it may not receive water from any other source than the clouds. 
In temperate regions, however, the amount of rain which falls is sometimes 
only a few hundredths of an inch or line; we therefore advantageously use a 
graduated tube, whose cross-section is a known aliquot part of that of the 
rain gauge, and into which the water from the greater vessel flows or is 
poured. If, for example, the diameter of the tube is one third that of the 
rain gauge proper, the water will stand nine times higher in the former than 
in the latter, which gives us a much better opportunity of ascertaining the 
value of small amounts. Conical gauges are also employed, yielding 
excellent results in slight rains. Self-registering rain gauges have been 
constructed quite recently, some of considerable excellence. 
P1. 29, fig. 1, is intended to illustrate the different conditions with respect 
to rain presented by different portions of the earth. The annual fall gene- 
rally diminishes as we recede from the sea, and increases with the héight of 
the place above the sea; at one and the same place, however, the amount of 
rain decreases with the height above the ground. Thus Dalton observed 
that the amount of rain on the top of a high tower compared with that at the 
bottom was as 2:3 in summer, and as 1:2 in winter. From nine years’ 
observation at the observatory of Paris, it was found that the amount of rain 
on one terrace was about 0.116 less than on another twenty-seven metres 
below. The reason of this difference is, that fresh vapor is constantly being 
condensed on the drop in its descent, and consequently the drops must be 
largest just before reaching the ground. ‘The nearer to the point of satura- 
tion the air happens to be, the more considerable is the difference just 
referred to ; for this reason it is less at Paris in summer than in winter. If 
it be very great, long continued rainy weather is to be expected. _This 
difference is not the same in all countries, and is Jess at Paris than in 
England. In warm countries, and in the warmer portion of the year, the 
rain is generally heavier than in cold countries and in the colder months. 
Between the tropics the rain sometimes falls to the depth of an inch in an 
hour, and Humboldt, in single instances in South America, observed from 
fowr to five inches in the same time. It is only very rarely that rain, to the 
amount of an inch in an hour, falls in higher latitudes. 
On the continents of the torrid zone a rainy season of many months’ 
duration sets in about the time of greatest heat, the heavens being clear 
during the rest of the year. When the sun is in the zenith the rain is most 
violent and copious. The duration of the rainy season is generally three to 
five months. Near the equator, where the sun stands twice in the zenith, 
and, indeed, on days which are separated by several months, there are two 
wet seasons, either separated by a dry one, or exhibiting a maximum in the 
amount of rain. Thus Dutch Guiana possesses a great rainy season from 
April to June, and a lesser une from the middle of December to the middle 
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