i8o THE POPULAR SCIENCE MONTHLY. 



record, two pages for each day ; the left-hand page is chiefly for 

 meteorological observations — the right, for miscellaneous events. 



At the end of every hour, both day and night, and in port as well 

 as at sea, the following items are observed by the midshipman of the 

 watch, and recorded in their respective columns : the speed of the 

 ship ; direction and force of the wind ; leeway ; height of mercurial 

 barometer and its attached thermometer ; temperature of the air and 

 of evaporation (dry -bulb and wet-bulb, both in a lattice-work case) ; 

 temperature of the sea at the surface ; weather by symbols ; forms of 

 clouds ; portion of sky clear ; condition of the sea ; and the sail the 

 ship is under. At the end of every four hours, the lieutenant in 

 charge of the deck enters on the right-hand page such particulars of 

 the weather as could not be described in the columns, together with 

 whatever events occurred during his watch. Every day at sea, the 

 navigator enters on the left-hand page the distance run since the pre- 

 ceding noon ; the latitude and longitude at noon, both by observa- 

 tion and by account ; the current (if any) experienced during the day ; 

 and the variation of the magnetic needle with the position in which it 

 was determined. 



The watches or tours of duty on board a vessel of war are divided 

 into four-hour periods, each watch being in charge of a lieutenant, as- 

 sisted by a midshipman ; the number of observers throughout the 

 twenty-four hours will, therefore, vary with the number of watch- 

 officers : generally there are four. 



Each lieutenant is solely responsible for the correctness of the log 

 during his watch ; but, as different officers contribute to the record of 

 a day, this lays the log-book open to both error and incongruity, if a 

 general supervision were not exercised by some one person. Such 

 is daily done by the navigator, who, after examination, certifies to 

 its correctness, and then the commanding officer examines and ap- 

 proves it. 



With accurate instruments, careful observers, and this system of 

 scrutiny, there remains nothing to be desired in the way of a continu- 

 ous, complete, and accurate record of the experience of a ship, whether 

 cruising on the high-seas or at anchor in a landlocked harbor ; and it 

 is believed that more trustworthy observations are never taken at sea. 

 Furthermore, they are made at such short intervals — every hour — and 

 the atmospheric phenomena and corresponding instrumental changes 

 are so closely contrasted side by side that no error, break, or flaw can 

 enter, without easy detection. 



I have been thus explicit regarding the log-books, in order that the 

 accuracy of the charts which are based upon them may be fully ap- 

 preciated. 



As regards the data furnished by merchant- vessels, in 1878 a very 

 complete meteorological journal was prepared at the Hydrographic 

 Office for the use of ship-masters, and is issued to them free of charge 



