TABLE.

Thermo

Feet.

meter.

32

35

40

45

50

55

60

65

The altitude thus found will be the altitude corrected, for the density of the air inversely as 85.86 the mean height of the barometer, at the two 87.49 stations is to thirty-six inches; therefore, 88.54 RULE. Multiply the mean temperature of the 89.60 two barometers (found in the table) by the tenths 90.66 of an inch in the difference of the two barometers, 91.72 and this product by thirty; divide the last pro92.77 duct by the mean height of the two barometers, 93.82 and the quotient will be the answer, or height re94.88 quired, with the error of a few feet only.

70

75

95.93

The height of the Peake of Teneriffe being measured by the barometer and thermometer.

70 mean height 'of the two thermometers, against which in the table stands 94.88, the mean temperature of the two barometers. Now, according to the rule, multiply 94.88 by 106, and the product by 30, and the last product by 25, and the quotient will be the answer, equal to 12,068 feet. This gives the Peake something more than two statute miles and one quarter, and a little more than two sea miles high, allowing 5,280 feet to one statute mile, and 6,000 feet to a sea mile. This nearly agrees with the measurement of the Peake, taken

in the year 1785, by the astronomers who accompanied La Pérouse on his voyage of discovery round the world.

70 mean height of the two thermometers,

against which in the Table, stands 94.88.

94.88

106

56928

9488

10057.28

30

25)301718.40(12,068 feet, and 1% of a foot.

25

51

50

171

150

218

200

If there should be but one barometer and one thermometer, they may be placed at the foot of the mountain, after the descent, for a few minutes, to give time for the temperature of the air and pressure of the atmosphere to have effect on the mercury in the instruments: then note the difference between the altitudes of both, at the beginning and at the end of the operation, and take the mean gravity and temperature of air during the operation.

The highest mountains in the world are the Andes, in South America, which extend nearly 4300 miles in length, from the province of Quito to the straits of Magellan, the highest of which is called Chimboraco; it is said to be 20,608 feet, nearly four statute miles, and nearly three and a half sea miles above the level of the sea; 2400 feet of which, from the summit, are always covered with snow.

Doctor Hamilton remarks, that the mercury is best cleansed by shaking it in a phial of fresh portions of water.

According to the calculations of different philosophers, the height of our atmosphere is about from 40 to 50 miles from

the earth.

The Hygrometer.

The Hygrometer is an instrument to measure the moisture and dryness of the air, and is formed of substances that will expand and contract on any alteration in the atmosphere, as it will receive humidity readily from a moist air, and part with it as readily to a dry air. In moisture it expands, and in dryness it contracts. On the contrary, cord, catgut, &c. contract by moisture, and expand by dryness. Many mechanical contrivances have been invented, to render sensible the smallest variations in the lengths of these substances. A description of one of them is as follows; and any person can make one for himself with very little trouble.

Let AB fig. 30, represent the section of the cylinder moveable out its axis, which is parallel to the horizon; at the end there is index I moveable against a graduated arc ab; about this linder some catgut tw is wound, one end of which is fixed the cylinder, and the other end to something at Z; now as e moisture of the air increases, the catgut contracts and turns e cylinder, and the motion of the index shows the increase of oisture, and as the air decreases in moisture the catgut will ngthen, and the weight of the index will carry the cylinder ck, and the index will show the corresponding decrease of oisture.

y

In order to make a perfect hygrometer, such substances must used as will contract or expand in proportion to the quanof moisture received. Mr. De Luc has made a great many periments, in order to find out such substances; and the relt is, that whalebone and box, cut across the fibres, increase ry nearly in proportion to the quantity of moisture received. e preferred the whalebone, first on account of its steadiness, always coming to the same point in extreme moisture; sendly, on account of its greater expansion, increasing in length ove one eighth of itself, from extreme dryness to extreme pisture; lastly, it is more easily made than any other.

Predictions of the Weather by the Hygrometer.

When the index of the hygrometer points and continues to treme dryness, fair weather, and probably wind, may be excted; but if the index returns to the mean state, it will rain. the index points to moist and increasing, rain will soon folw; if it returns towards the mean, it will be fair weather. I have tried many different kinds of hygrometers, but have

frequently been deceived. I think that from its quick sensibility to receive moisture, and as readily to part with it on the least change in the air from moist to dry, this instrument may be a disadvantage rather than an advantage, as any substance which may be colder than the surrounding atmosphere, will find the moisture of the surrounding air condensed on this substance; and by that means imbibe moisture; and on the other hand this substance may also, by some cause, become warmer than the surrounding atmosphere, and by this means the moisture in the atmosphere will be absorbed, and by such accidental cause point the index to the wrong degree, &c. It will be on the same principle that I have seen a glass tumbler filled with cool pump water, in Philadelphia, in summer, when the moisture of the surrounding atmosphere being much warmer than the glass, become condensed on its surface, and the outside surface become quite wet; which leads some to think that the water penetrates through the pores of the glass. But if the glass had been filled with water warmer than the surrounding atmosphere, the moisture round the surface would have been absorbed by the glass. However, the use of the hygrometer on board ship being in its infancy, I have no doubt that in time, with long practice, some kind of substance may be found to obviate the defects just mentioned. But it is best not to depend too much on its indications.