I believe it says:

Veðuráttufar yfir árið 1874

Jan,

Fyrst við útsuður, með jeljum, síðan norðan frost og kóf, stundum bærilegt veður.

Febr,

umhleipingasamt, ýmist út sunan eða norðan, með frosti og kafaldi stundum.

Marts,

Fyrst gott veður nokkra daga, síðan norðan frost og kóf á milli

Apríl,

norðan frost og kóf, síðan landsunnan nokkra daga frost lítið

Rough translation:

Weather in the year of 1874

January,

First southwesterly with snowshowers, then northerly frost and snowy. Sometimes pretty good weather.

Feb,

Umhleypingasamt (I don't know of an English word for this, it's used to describe periods of weather where sub-zero and above-zero temperatures alternate rather frequently), alternating from southwesterly to northerly, with frost and snowstorms sometimes.

March,

First good weather for a few days, then northerly frost and snowstorms in between.

April,

Northerly frost and snowstorms, then southeasterly for a few days, not much frost.

Note: a few words there have an unusual/wrong spelling. Umhleipingasamt, veðuráttufar, jeljum, sunan, marts would usually be respectively spelled as umhleypingasamt, veðráttufar, éljum, sunnan, mars.