226 Longitude by Moon's Altitude. 



March 



and especially from July to October, noticed by the casual ob- 

 server, should be less than in *01, and therefore approximate to 

 the actual average for July, which as we have seen is probably 

 something less than in, 01. The agreement between theory and 

 observation is therefore as close as it could reasonably be expected 

 to be, in a case in which there is an uncertainty, within certain 

 limits, in reference to some of the numerical results arrived at 

 inferentiaily, and in which the observations have not all that 

 completeness and definiteness which is essential to a thorough 

 testing of the theory. This is seldom realized except where the 

 bservations are made for the express object of subjecting a 

 heory after it has gained a foothold in the region of science, to 

 the most rigid scrutiny. 



I conclude, therefore, that the heat evolved from the dew, or 

 condensed vapor, that falls at night, is nearly, if not quite suffi- 

 cient to reduce the theoretical decrease of temperature due to ra- 

 diation, down to the amount which actually obtains ; and that 

 the variations in the quantity of dew that falls at night, from one 

 season to another, are attended with sufficient variations in the 

 amount of heat imparted to the earth, to effect the changes ob- 

 served in the nocturnal decrease of temperature during the year. 

 As to the general explanation of the effect of dew, I conceive 

 that, in the average of months, the amount of dew deposited 

 from hour to hour during any one night, and from night to night, 

 is in proportion to the humidity of the air, and therefore must 

 increase steadily from sunset to sunrise, and from summer to 

 winter. I consider that it has been conclusively established, that 

 it is only from the varying amounts of heat imparted to the earth, 

 consequent upon these varyiug quantities of dew, that the ob- 

 served changes in the nocturnal losses of temperature from hour 

 to hour, and from night to night, can possibly result. 



(To be continued.) 



Art. XVII. — On the Method of determining the Geographical 

 Longitude by Altitudes of the Moon ; by W. Chauvenet, Pro- 

 fessor of Mathematics in the U. S. Naval School. 



This method has been proposed for the use of navigators on 

 account of its practicability with the sextant, and the ease with 

 which altitudes are observed with that instrument. It can be of 

 little use, however, at sea, where the uncertainty of the data 

 the local time, the latitude and the moon's altitude — is such that 

 the result of an observation of this kind could rarely be relied 

 upon within 30' of longitude. 3 * But on land, with the artificial 



Raper (Practice of Navigation, p. 285) says % of a degree. 



