54 Professor Carpenter on the supposed Temperature 



suits differ not more from the mean temperature of the place, 

 than from each other. These discrepancies, or rather these 

 departures from accurate results, are dependent on many 

 circumstances. In the first place, thermometers of very in- 

 ferior quality are in very general use, and tliey will often differ 

 in the results afforded by four or five degrees. In the next 

 place, sufiicient importance is not attached to the position of 

 the instrument ; and instead of having it completely protected 

 from reflection, and in almost complete obscurity, taking care 

 at same time to secure a free circulation of air about it, we 

 find them often in such a situation as to receive from walls or 

 the earth, a considerable portion of reflected heat ; often 

 in closed apartments or against walls, the temperature of 

 which is influenced throughout the day, by the full force of 

 the sun. Galleries fronting the north are favourite places for 

 suspending thermometers, under the impression that no heat 

 is reflected from that side. In this way we often find, in this 

 climate (the United States), that when a thermometer proper- 

 ly placed gives a temperature of 90° or 91°, all the others in the 

 vicinity will stand at 95°, 100°, or even 110°; so that the annual 

 mean derived would be very greatly above the true one. There 

 is, however, it appears to me, a source of error much more gene- 

 ral. I speak of the methods of calculating the mean after the 

 observations have been made, and of the adoption into gene- 

 ral use of methods which have been found to give correct re- 

 sults at particular places. The methods most in use are the 

 following, viz., to take three observations, and from these cal- 

 culate the mean directly ; some fix upon sunrise, 2 P.M., and 

 sunset ; others, as in the Army Meteorological Register, 

 and in those of most of the meteorological societies, upon 7 

 A.M, 2 P.M., and 9 P.M. I have ascertained, by taking the 

 mean of hourly observations made here by myself, that any of 

 these methods give means which, during every season of the 

 year, are too high, and that the error in excess is greater, in 

 proportion as the diurnal exceeds the nocturnal temperature. 

 The last method, which is now in most general use, has pro- 

 bably been adopted in colder climates, as agreeing experimen- 

 tally in its results with those derived from more frequently 

 repeated observations during the same period ; or possibly 



