184 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



but it is a thousand times easier to learn to make barometers than to learn 

 to take altitutdes with one with any considerable degree of accuracy. 



"The marine barometer has to be more expensively constructed to obvi- 

 ate the oscillation of the mercury which would otherwise be occasioned by. 

 the motion of the vessel ; but the essential principle of action is the same 

 in both, and is quite superior to the principle involved in the action of the 

 Aneroid, Union, or any other. It is the principle discovered by Torricelli, 

 over two hundred years ago, that, at the level of the sea, a column of mer- 

 cury about thirty inches high will ordinarily balance the pressure of the 

 air upon its base — the air not being permitted to press upon its top at the 

 same time; and, when the air is heavier than ordinary, it will support a 

 little more than thirty inches, and when lighter than ordinary, less than 

 thirty, &c., so that it will be readily perceived by any one at all acquain- 

 ted with the principles of natural philosophy (as all should be in my esti- 

 mation), that this cannot be simplified, and all attempts at greater accu- 

 racy have failed with anything not too cumbrous for ready transportation. 

 A liquid barometer thirty feet instead of thirty inches high, would, of 

 course, have twelve times the range of the mercurial, and act on exactly 

 the same principle, and have a corresponding delicacy or sensitiveness; 

 but it would be somewhat inconvenient, though not so much so as might 

 ■at first sight be imagined, as it could be arranged to examine at the bot- 

 tom instead of the top. The old Torricellian barometer, while the most 

 durable and accurate, I think must be the cheapest to get up of any kind 

 ahat is good for atiything. But the makers of those now sold have to pay 

 for a patent on them, but which does not affect their utility one way or the 

 other, but merely relates to an arrangement to render them portable without 

 the danger of getting air into their tubes, which would render them useless 

 till that was discharged. The portability apparatus is entirely out of use, 

 except during transportation." 



Mr. Solon Robinson. — This communication involves an important ques- 

 tion, and one that this Club should carefully consider. Can we recommend 

 these instruments to farmers as valuable weather indicators ? According 

 to my experience I should say no — that to any but well educated men, who 

 have leisure to study and compare, a bai-ometer is of no practical advan- 

 tage. 



Prof. Mapes. — The barometer at sea is a very valuable instrument; on 

 the land it is of very little use. I have an hygrometer that I have used for 

 the last fifteen years. It is made of two kinds of wood, one lengthwaj^s of 

 the grain, the other across the grain. This instrument is nearly always in 

 motion as the air is damp or dry, and by noticing the movement you will 

 soon be able to understand it. 



RULES FOR THE USE OF BAROMETERS. 



Mr. Solon Robinson. — To those who have barometers and leisure to study 

 and apply the rules, the following will be useful. Prof. Silliman gives the 

 following rules, which embody the results of long and various experience 

 in different places: 



" 1. When the mercury is very low, high winds and storms are likely to 

 prevail. 



