224 On the Comparative action of Aneroid Barometers. [No. 3. 



That Inertia is every where present and must always be first over- 

 come, no one acquainted with the laws of physics will question, as also 

 that it resists motion at all times, and on the minutest as well as on 

 the largest scale. Now considering first the common Marine Barome- 

 ter (the sluggish Barometer as Mr. Dent* somewhat unfairly terms 

 it), we have here, the Inertia of the column of mercury and its attraction 

 of cohesion, and then — and in dry weather and with badly prepared 

 leather, this may not be trifling, the Inertia (want of elasticity) of the 

 leathern bag in which the mercury is inclosed in the box, or of the 

 leathern bottom to it, to overcome, before a minute atmospherical 

 variation can affect the column. Our practice of gently tapping the 

 Barometer before reading off is the familiar recognition of the exist- 

 ence of all these obstacles to the free motion of the mercury. 



In the Aneroid we have the Inertia of the plate covering the vacuum 

 vase, and then the Inertia and friction of a train of machinery levers, 

 &c. to overcome before we move the hands ; and when we recollect 

 that, even with the powerful spring of an eight-day Chronometer, 

 the balance must, in sailor language, " get a start" by the semi-circular 

 motion which must be given to it to set it going when it has been 

 wound up after being let down, as every one who understands the 

 management of Chronometers is aware, we can form some idea of what 

 the Inertia and friction of the machinery of the Aneroid, trifling 

 though it be, amounts to. 



In the Simpiesometer we have only the Inertia and attraction of 

 cohesion of the small column of oil, or acidf to overcome ; the whole 

 of which probably does not exceed that of an inch of the Barometric 

 column ; for, as the atmosphere acts directly upon the surface of the 

 liquid in the curved tube, all that Inertia which arises from the greater 

 or less flexibility of the leathern bag, and the great weight of the 

 mercurial column being avoided. In the fixed standard Barometers 

 this direct action upon the surface of the mercury is also allowed to 

 take place, but the weight of the column still remains. There may 



* Treatise on the Aneroid. 



t Some Simpiesometers are said to be filled with an acid, but it is difficult to 

 imagine what kind of acid would not either evaporate or absorb water, unless as I 

 suppose, the top is covered with oil. 



