jQuery Grid Plugin - jqGrid

I stuck as closely to the myfirstgrid and demo examples as possible in the relevant code:

function populateTable() {
jQuery("#timeline").jqGrid({
    datatype: "local", 
    height: 250, 
    colNames:['Start','Symptom','Duration'], 
    colModel:[ 
        {name:'start',index:'start', width:60, sorttype:"int"}, 
        {name:'name',index:'name', width:90, sortable:false}, 
        {name:'duration',index:'duration', width:100, sortable:false} 
    ],
    multiselect: false, 
    caption: "Event Timeline" 
});

for(var i=0;i<=globalArray.length;i++) {
jQuery("#timeline").jqGrid('addRowData',i+1,globalArray[i]);
};
}

With a blank table

<table id="timeline"></table> 

in the corresponding html.

When the function is called, the second line of code throws the error: "Error: Object doesn't support property or method 'jqGrid'" at line 2, above.

Any help would be appreciated.