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differ 9°.5. In 7 and 17, the dew-points are necessarily nearly 

 the same. The results got by the formula differ by only 

 l°.2, while the instrumental ones differ by 6°.6. Again, 

 2 and 24 should give g.p. the same dew-point. This is true of 

 the formula, but not at all of the hygrometer, as the tem- 

 peratures it yields differ by 2° .5. The preceding instances, 

 the number of which might be greatly augmented, were. Dr. 

 Apjohn conceived, quite sufficient to show, that even in the 

 hands of Captain Boileau, Daniell's instrument has not given 

 correct conclusions ; and that, therefore, generally it cannot 

 be relied upon for determining, accurately, the hygrometric 

 relations of the atmosphere. 



With respect to the column of results obtained by Cap- 

 tain Boileau, from the wet bulb process, Dr. Apjohn could 

 not entertain the slightest doubt of their exactness, having 

 found that his formula stood the severest experimental tests 

 to which he could subject it. These experiments, however, 

 (see Transactions Royal Irish Academy, vol. vi.) were, he 

 admitted, all made under pressures, at or about 30 : and 

 hence it always appeared to him desirable, that they should 

 be repeated at such diminished pressures, as are met with 

 at elevated points on the earth's surface. In this point of 

 view, the Simla observations appeared at first highly impor- 

 tant, the pressure there being but little over 23 ; but as no 

 reliance can be placed on the dew-points directly got, they 

 cannot be used to test the accuracy of the hygrometric ex- 

 pression. 



Dr. Apjohn then expressed a hope, that no one who 

 heard him would misunderstand him to assert, that the dew- 

 point could not be accurately got by Daniell's instrument. 

 He knew it could, and he had explained elsewhere how to 

 use it, so as to arrive at a correct result. What he had, 

 however, asserted before, and would again repeat, was, first, 

 that it was an instrument very difficult to observe accurately 

 with ; and second, that when Mr. Daniell's rule is attended 



