1821.] Dr. Apjolm on Wollaston's Thermometer. 295 



barometrical determinations are rather surprising, the deviations 

 beino- much within the limits of the possible errors of observation. 



On the whole, this instrument appears to me much more eli- 

 gible than the common barometer. It has many advantages 

 over it. I will enumerate some of them. In the first place, it 

 is much more portable — a circumstance of considerable conse- 

 quence, and one Avhich will be duly appreciated by those who 

 have had sufficient scientific zeal to carry one of Troughton's 

 barometers to the summit of a high mountain. Secondly, it is 

 capable of being made much more sensible. The sensibility of 

 the instrument with which the above observations were made is 

 to that of the barometer nearly in the ratio of 65 to 48. Thirdly, 

 the calculation is much more simple, as is obvious from the 

 examples already given. The table precludes the necessity of 

 having recourse to logarithms ; and it is an advantage peculiar 

 to this method, that it is encumbered but with one of the 

 two corrections necessarily embraced in barometric formulas. 

 Fourthly, an observation is made with surprising despatch ; for 

 as the instrument should be always kept in the steam, on 

 account of the superior equability of its temperature, a very small 

 quantity of water in the boiler will be quite sufficient for every 

 purpose. 



Dr. Wollaston does not appear to have used his instrument 

 unless in subservience to the barometer. He merely divides an 

 attached scale into a number of equal parts, and knowing the 

 number of these corresponding to a degree of Fahrenheit, and 

 the altitudes of the barometer at the times of the two observa- 

 tions, he is enabled to calculate the true height. This attention 

 to the height of the barometer by a second observer is generally 

 found very inconvenient, and is, I believe, quite unnecessary, 

 unless when the weather is very unsettled, or the interval of time 

 between the observations considerable ; or, lastly, where extreme 

 precision is desirable. To render the thermometrical barometer 

 an independent instrument, and even superior in consequence of 

 its greater sensibility to any single barometer, it is only neces- 

 sary that the point 212 be taken for a pressure of 30, which may 

 be done, under any given pressure, in the following manner : 

 Take the difference between the height of the barometer at the 

 time and the standard height 30 ; and as 3 is to 5, so will this 

 difference be to the distance (measured in degrees of the scale) 

 above or below the point at which the instrument then stands, 

 at which 212 is to be placed. This fourth proportional is to be 

 measured upwards if the barometer stands below 30, downwards 

 if above 30. The practice of this method evidently presupposes 

 the knowledge of the length of a degree in the scale of the 

 instrument, and this should be determined (as Dr. Wollaston has 

 judiciously remarked) at a low temperature before the instru- 

 ment is sealed, by comparison with a good thermometer. 



