Precautions To Ensure Correct Graduation

Those who possess a boiling-point thermometer should satisfy themselves that it has been correctly graduated. To do this, it is advisable to verify it with the reading of a standard barometer reduced to 32° F. The table of “Vapour Tension” (given at p. 62) will furnish the means of comparison. Thus, if the reduced reading of the barometer, corrected also for latitude, be 29·922, the thermometer should show 212° as the boiling-point of ater at the same time and place; if 29·745, the thermometer should read 211·7; and so on as per table. In this way the error of the chief point of the scale can be obtained. Other parts of the scale may be checked with a standard thermometer, by subjecting both to the same temperature, and comparing their indications. The graduations as fixed by some makers are not always to be trusted; and this essential test should be conducted with the utmost nicety and care.

Admiral FitzRoy writes, in his Notes on Meteorology:—“Each degree of the boiling-point thermometer is equivalent to about 550 feet of ascent, or one-tenth to 55 feet; therefore, the smallest error in the graduation of the thermometer itself will affect the height deduced materially.

“In the thermometer which is graduated from 212° (the boiling-point) to 180°, similarly to those intended for the purpose of measuring heights, there must have been a starting point, or zero, from which to begin the graduation. I have asked an optician in London how he fixed that zero, the boiling-point. ‘By boiling water at my house,’ he replied. ‘Where is your house?’ In such a part of the town, he answered. I said: ‘What height is it above the sea?’ to which he replied, ‘I do not know;’ and when I asked the state of the barometer when he boiled the water, whether the mercury was high or low, he said that he had not looked at it! Now, as this instrument is intended to measure heights and to decide differences of some hundred, if not thousand feet upwards, at least one should endeavour to ascertain a reliable starting point. From inquiries made, I believe that the determination of the boiling-point of ordinary thermometers has been very vague, not only from the extreme difficulties of the process itself (which are well known to opticians), but from the radical errors of not allowing for the pressure of the atmosphere at the time of graduation—which may be much, even an inch higher or lower, than the mean, or any given height—while the elevation of the place above the level of the sea is also unnoticed. Then there is another source of error, a minor one, perhaps: the inner limit, the 180° point, is fixed only by comparison with another thermometer; it may be right, or it may be very much out, as may be the intermediate divisions; for the difficulty of ascertaining degree by degree is great: and it must be remembered that the measurement of a very high mountain depends upon those inner degrees from 200° down to 180°, thereabouts. Hence, the difficulty of making a reliable observation by boiling water seems to be greater than has been generally admitted.”