VOL. LXVII.J PHILOSOPHICAL TRANSACTIONS. 275 



To make use of this table, seek the height which the barometer is found to 

 stand at in the left-hand column, if the boiling point is adjusted either in the 1st 

 or 3d method, and in the 2d column if it is adjusted in the 2d method ; the cor- 

 responding number in the 3d column shows how much the point of 212'' must 

 be placed above or below the observed point, expressed in thousandth parts of the 

 interval between the boiling and freezing point : for example, suppose the boil- 

 ing point is adjusted in steam when the barometer is at 29 inches, and that the 

 interval between the boiling and freezing points is 1 1 inches ; the nearest 

 number to 29 in the left-hand column is 29-03, and the corresponding number 

 in the table is 7 higher, and therefore the mark of 212° must be placed higher 



than the observed point by -^ of the interval between boiling and freezing, 



that is, by , or .077 of an inch. This method of correcting the boiling 



point is not strictly just, unless the tube is of an equal bore in all its parts : but 

 the tube is very seldom so much unequal as to cause any sensible error, where 

 the whole correction is so small. The trouble of making the correction will be 

 abridged by making a diagonal scale such as is represented in fig. 15. It is not 

 very material what kind of water is used for adjusting the boiling point, so that 

 it is not at all salt ; only, if any kind of hard water is used, it is better that it 

 should be kept boiling for at least 10 minutes before it is used. But we would 

 advise all those desirous of adjusting thermometers in the most accurate manner 

 for nice experiments, to employ rain or distilled water, and to perform the ope- 

 ration in the first mentioned manner, that is, in steam. 



On the freezing puiiit. — In adjusting the freezing, as well as the boiling point, 

 the quicksilver in the tube ought to be kept of the same heat as that in the ball. 

 In the generality of thermometers indeed, the distance of the freezing point 

 from the ball is so small, that the greatest error which can arise from neglecting 

 this precaution, is not very considerable, nnless the weather is warmer than 

 usual ; but as the freezing point is frequently placed at a considerable distance 

 from the ball, the operator should always be careful either to pile the pounded 

 ice to such a height above the ball, that the 

 error, which can arise from the quicksilver in 

 the remaining part of the tube not being heated 

 equally with that in the ball, shall be very 

 small ; or he must correct the observed point, 

 on that account, according to the annexed 

 table : 



Tlie 1st column of this table is the heat of the air, and the 2d is the correc- 

 tion expressed in 1000th parts of the distance between the freezing point and the 

 surface of the ice : for example, if the freezing point stands 7 inches above the 



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