302 Remarks on Self-Registering Thermometers. 



while the instrument is held in this upright position, it must 

 be so held until the mercury has resumed the true temperature 

 of the air, — an operation of so much nicety, and of such consi- 

 derable time to be properly performed, that I do not think it 

 a very advisable postulatum in the adjustment of any instru- 

 ment. If the instrument is delicate, the very heat of the hand 

 or the body is sufficient to prevent this, and if it be clumsy, the 

 difficulty is only tenfold increased. Another imperfection I 

 should foresee is in the method by which the column of mer- 

 cury is cut short, namely, by its own gravity as it reaches the 

 orifice of the tube. Now, those who are accustomed to the 

 operations of this fluid must be aware of the effects of its great 

 density and cohesion of parts, and that when pressed from the 

 mouth of a tube of any kind it neither trickles off like a fluid 

 of less specific gravity, nor falls off exactly where the tube 

 ceases its support, but that it forms a globule at the mouth of 

 very considerable size, which does not fall till the gravity of 

 the mass exceeds the cohesive attraction it experiences to the 

 remainder of the column. Hence it is clear, that at one time, 

 if the temperature reaches the maximum, and begins to decline 

 just as a drop has fallen from the mouth of the tube, the zero will 

 be correctly at that point, but at another the mercury in the 

 bulb may begin to contract just as the globule has reached its 

 greatest size, when, instead of dropping off, it will gradually 

 retire again into the tube, causing an error, which might, I 

 presume, be frequently considerable. In Mr Blackadders 

 idea, however, of a register without any index, there is great 

 ingenuity ; and in the case he proposes for registration at a 

 particular hour, these objections do not hold, but in my opi- 

 nion are replaced by others still more serious. It is impossible 

 to conceive that an instrument should be used except by one 

 or two individuals, which requires not merely a time-piece at- 

 tached, but a whole apparatus of a perpetual burning lamp in 

 one case to keep it warm, and an evaporating contrivance in 

 another to cool it.* Such complexity ill suits with the present 

 state of meteorological science, which finds few enough persons 

 who will pay a due regard to the accurate use of the usual 

 means of information ; and there are even some who, like Mr 

 • See this Journal, No. vii. p. 251, and No. ix. p. 93. 



